We had to go through their approval process to be able to schedule things on your behalf. Any platform that does not ask for your username and password is legit and has been approved by Meta or Twitter etc. Some platforms ask you to install a chrome plugin to connect your account, those are not legit.
I believe they dont have API access to Instagram and just send you a reminder to post something when you schedule it. Kinda lame
LinkedIn provides similar access, as well as Twitter, and other social networks.
There are literally hundreds of companies that provide functionality similar to Buffer and Hootsuite, including scheduling for future publishing which is not forbidden at all. (Source: I run one of these services.)
The confusion may arise because until 2021 it wasn't possible to publish directly to Instagram. Before that only some big players like Hootsuite and Buffer had direct access to the publishing api, and some less than reputable companies used unsanctioned hacks to schedule Instagram content. But in 2021 Instagram opened their publishing api to other third party apps.
[0] https://developers.facebook.com/docs/pages/publishing/
[1] https://developers.facebook.com/docs/instagram-api/guides/co...