I think it boils down to actual intentions. Being furloughed sucks and when I got hired after my furlough from a major, I had absolutely no intention of going back to airline #1. I resigned and they still called when the time came and I told them TBNT. That's reality. I think in the current environment of pilot shortage, it's reasonable to assume that Airline X is still going to try to get their furloughees to come back no matter what letter they send to get hired elsewhere.
If a furloughed pilot comes to FedEx with a reasonable intention of making it a career, that's all one can ask. If they decide when the time comes that their former life suits them better, then it's a business decision.
The only problem I have is with the ones who are simply using FedEx as a placeholder until they can go back to mama. If they know they have no intention of staying, that's a pretty low move. Especially because it's very likely that they will taint the waters for every furloughee who comes after them looking to make a change after they realize the value and stability a cargo career offers when faced with yet another unexpected industry challenge.