In general, defamation lawsuits are very hard to win. You have to PROVE that the person in question 1. Made a false statement regarding you. 2. It harmed you in some meaningful way (significant loss of profit.) 3. That the person in question made the statement knowing it was false, or made the comment completely negligently.
Seeing how the comment was made about Amazon's platform, not you personally as a contract worker, you don't have much of a case. Amazon could potentially say that the comment hurt their profits, but would probably struggle to prove it was completely untrue (multiple submission from the same IP), or that the comment was made negligently.
I'm definitely not an expert at this kind of thing either so take everything I say with a grain of salt.