It happens all the time. Even with things that seemingly arrive 'out of the blue'.
Both the Germans and British invented jet engines within months of each other. They also both invented radar at around the same time.
I have invented things that were later patented by others, too. This was my most important, I invented it about 10 years before it was patented, but (stupidly) did nothing with it: https://www.youtube.com/watch?v=CjTa3SeKuaE
Millions of people have the same background knowledge that you do. Any one of them could have done the same as you. The thing about one-in-a-million chances is that when you have ten million opportunities you will very likely get ten hits.
When did you actually see that it was filed? If you saw it come out of the patent office a few days later, it was probably filed much earlier than your forum post.
If you saw it later but it was filed several days after your post, I think it is feasible, perhaps with the aid of someone like GPT-3, to put together a patent application that quickly.
They may also have been working on the same thing at the same time as you, and then needed to accelerate their patent application after your MVP posting.
The only thing that really matters is the claims section of the patent, so if you can access those be sure to read them to determine how much it overlaps with your stuff.