While sort of true, using the word "proof" there is too strong. In a civil context, the burden of proof for a retaliatory firing is a preponderance of the evidence. That means, the plaintiff has to demonstrate with evidence to the court (in a bench trial) or the jury that it is more-likely-than-not (e.g. 51%) that the firing was retaliatory.
If you start with the evidence that Amazon learned that the worker was organizing a strike, and then very shortly thereafter fired the worker that evidence _alone_ (which seems to be undisputed) probably gets you near that burden.
Amazon, then, might present the lack of quarantine defense as an alternative scenario, but then some of the burden will be on Amazon to effectively make this case.