Disclaimer: I currently work at Amazon as an SDM (I do not speak for the company). I've also worked a lot of other places. In my experience, bad managers can happen anywhere.
I'm not going to take sides since all we have is a story told by an ex-employee who has every reason to be spiteful toward his former employer. The other side of this story is one we'll likely never hear.
I do want to comment on the visa aspect of this (sounds like he's on an EB3). I have worked with and managed visa-holding engineers who've been passed over for promotions or otherwise discriminated against because of their visa status (not at Amazon). Certain kinds of promotions can run the risk of a visa holder getting their visa rejected. Many things a company might do to improve the circumstances of a regular employee runs the risk of triggering a visa re-evaluation. More often than not the company will play it safe so the employee doesn't run the risk of getting deported. That can involve denying them promotions or "unofficially" giving it to them (all the responsibility but no title). It's systemic discrimination, and the worst part is it has the "best interests" of the visa holder at heart.
Personally, I tried to counter this kind of discrimination by making the employee aware of the situation and the risk, then asking what they would like me to do. Usually, though, my influence is limited and I get blocked by HR and legal.
If this story didn't happen to a visa holder, it would be a simple story of "just go somewhere else," but I can understand how helpless these people must feel.