It's not an explanation, it's an excuse. Planes are running completely full, there is no reason a monorail can't. And by no reason, I mean no legal or regulatory restriction blocking Disney from doing so. At the very least, they could run at 50% capacity, which since the Poly and Contemporary tower are closed, should be sufficient.
Disney is now filling theaters leaving only a space or two between parties. Gondolas have multiple parties per cabin. I accept the fact that the Epcot monorail can't run so long as Disney chooses to only run the monorails at 20% capacity. I do not accept their choice to do so as rational or necessary, however.
It's basically my wife saying hey, let's go for a run, and I say I can't because I'm sitting on the couch and don't have my running shoes on. Technically, that's true and explains why I can't run. But the explanation is based on a faulty premise that I can't do something whereas the reality is I'm just choosing not to.