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.