But if the plaintiff produces no evidence, Amazon does not need to make a defense. Thus OP is correct.
Sure, but any evidence which makes an accusation more likely than in the absence of that evidence suffices to meet preponderance of the evidence in the absence of any contrary evidence. The fact of the labor organizing, the fact of the firing, and their temporal relationship are, together, evidence for retaliation.
So, Amazon will very likely need to make the case (and Amazon will need to present the evidence to support it), that he was actually fired for violating the company mandated quarantine.
The actual evidentiary fight will probably be over whether that quarantine was a bona fide quarantine, or a pretextual one. But who has the burden to present that evidence will very much depend on who feels like they're losing the case. Probably both of them will need to present evidence to support their position.
You have a reasonable indication, but no preponderance of evidence. You probably have enough for discovery.