When you sign on at a FAANG - you get a sign on package. This includes stock grants that vest over time. After that time runs out, if they like you, they will promote you and give you a new batch of grants. If you don't think you are great, no promotion and no new stock grants. So while you are not demoted, your actual pay drops sometimes significantly.

There is a good reason for this. It allows them to drive increased turnover at higher levels in the org where it's normally hard to transition folks out. They just don't refresh equity or promote with a new equity grant.

Anyways, the poster did what anyone should do - quit and find somewhere else to work.