During the "Show HN" some bugs are discovered and issues with scaling, but overall the feedback is extremely positive. So they know, that it is worth working on and continue to make it a proper product (dealing with scaling, privacy, security, pricing). Since they are a sole developer it will take time to do this.
Wouldn't it be possible that Skype, Whatsapp, Zoom or some other actor in that area would copy their idea? Given that they have explained on some level how it works in the "Show HN". In addition, now that it has been shown to be possible, the big companies can assign a team to reverse engineer it.
Two weeks later, the original developer has fixed the problems and launches the product successfully. A few days later Skype introduces a new video chat feature that essentially does the same thing. No one bothers to switch to FastChat, since it doesn't have any outstanding features.
Is this scenario likely? Can it be dangerous to post your MVP on "Show HN"?
Also, even if FastChat was the only app with your new feature (maybe because you were able to patent it), that still wouldn't guarantee success. Skype, Zoom, etc. would already have a huge base of customers while FastChat would be starting with none, so they'd have the advantage of network effects (a FastChat user would first have to convince someone they wanted to talk to to install FastChat). They'd also have much more resources to throw at alternate approaches to making their apps faster, and they'd probably be much more reliable than FastChat because they'd have large operations staffs to keep their servers running under high loads. Corporate users would probably want to stick with a service with a proven track record, since this kind of service is now mission-critical infrastructure for them.
[1] https://www.investopedia.com/ask/answers/05/economicmoat.asp
The short and obvious answer is: sure, it can be.
You might get cloned by someone with an advantage you don't have at the moment, before your product can get the traction it needs to become sustaining. It's far easier (in all respects) to clone something in the earliest stages than to go through the painstaking, time consuming process of working through all the twists and turns it takes to figure out how X fits with Y. It's a weird, complex problem - when and who to show a thing to - that you're unlikely to find an easy answer to, it really can't be modeled perfectly. You don't know who is out there reading HN that might kick your ass. That's a coin flip. This site is filled full of technical talent that can rapidly clone anything you've produced, so if they have any other advantages that you lack, you could certainly be helping someone take you out.
Some people here will retort that that just means you were doomed anyway. That doesn't inherently follow. Getting an early momentum advantage such that you get some good entrenchment before the clones attack, can make all the difference, even if someone comes at your product with more capital to spend than you have for example. To the extent you get any entrenchment, you become that much harder to displace easily.
The ideal is to deliver your product to the audience that wants it. If the technical audience on HN are not the best audience for your product, do not present it to them.
Also, I think that the probability of this happening is possibly lower than you think, as it assumes that your posting is wildly popular AND popular enough that someone at a big tech co. thinks it's worth building AND that person is convincing enough to convince his team to build your product AND they're able to move as fast as you're saying AND they execute well enough AND there is not room enough for more than one co. in the market you're after.
Even if you do manage to keep things a secret and choose a different path, who is to say that the alternative path doesn't somehow end up exposing you even more due to the large number of unknowns that you'll encounter as a founder?
You may even be able to move faster than Zoom as a solo dev depending on the circumstances. In the long run, I really don't think that a posting to HN will destroy a co. Most of the time it will be something else like lack of motivation, time, resources, etc.
Technical moats are hard to pull off when you're small. I'd suggest weaving together enough features that no one will copy the specific manner or style in which you've weaved them together. If the success or failure of your product depends on the novelty of a single feature, then it's not properly prepared to slay copycats.
Ideological moats however work even with a solo founder. If you're building something that's easy to copy, you're going to take huge morale hits when other people inevitably implement the same features. You might stop and question your motivations then, or decide to quit. Often times what kills you isn't the competition, but rather your thinking that the competition constitutes a problem in the first place.
This is what ideological moats are for. No matter what anyone else does, you're going to continue working. You can't be killed. The reason for this should be that your product isn't merely the sum of its parts, but is an ideal you're working towards. This might not be true, but you need to believe it anyways.
Ordinarily though, to make money you will have to release the product, and competitors as well as customers will now know about you.
If your entire implantation took a short time, then it will be easy enough to clone.
To avoid cloning by a major company, you should pick an implementation that goes against one of their core user experience elements. For example, Slack began as a chat service for small working teams, and only small teams. Other chat services existed but they let you talk to anyone, or they were pure customer-service chat. Those competitors found it hard to integrate any ideas from Slack because doing so would essentially weaken their existing product.