So, sometimes (before or during the pandemic) an airline cancels an underbooked flight. and sometimes they may invent a reason to try to reduce customer rage. Many people already assume (rightly, I think) that some fraction of “mechanical problem” cancellations are actually an airline trying to reduce the number of flights to save money.
However, airlines also use many other strategies to avoid losing money that way, and in many cases, it is not a choice they would make, because it’s not just passengers they need to get from place to place – it’s also airplanes, crew, cargo, etc., and a plane changing its plans could mess up their whole schedule of operations.
So not only is it not often they choose to disrupt their schedule, but they don’t need Covid-19 to invent an excuse, and Covid-19 would be a panic-inducing kind of excuse to make up. One of the last things the airlines want, is to increase the number of conversations anyone has about the risk of catching deadly diseases on an airplane! And there is no reason to choose to tell that lie to cancel a flight.
As for current refund policy, when someone needs to cancel a flight for Covid (usually because they’ve caught it or need to quarantine), the airlines have been allowing rescheduling without any of the usual rescheduling fees. Again, this is part of trying to reduce the apparent impact of Covid on air travel.