Meta provides direct API access for publishing content to Facebook pages [0] and, more recently, to Instagram accounts [1]. Any third party company can apply for access and use these apis as long as they comply with Meta's terms of service and data security requirements, which have become stricter in the last few years (for each individual api permission you must submit documentation and evidence of how your app will use the permissions, and must provide a way for Facebook to log in to your app and test the functionality).

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...