Isn't it more likely that after shipping a 'named' consumer product, many of those engineers rose in value a tremendous amount and rather than negotiate a raise they took a huge salary bump working for Apple/Google?
In my experience, companies are not that great at recognizing and rewarding achievement. So unfortunately, I'll stick to my cynical interpretation.
Also, we're talking about dozens or even hundred of people moving on from Amazon here...
Google and Facebook offer largeish stock grants when you join the company, no ? Google is also known, to offer some kind of bonuses on merit, but that policy is seen as remarkable in tech.
I'd guess Amazon is more political and/or stingy than the above 2.
Anyway I was not referring to big tech specifically.