Not american here, so didn't read/listen to this, but in all honesty, I would read the last part not as 'calling' the corona virus a hoax, but that democrats were using the corona virus as an attack, as they did with russia or the impeachment (not passing judgement about russia/impeachment, just I would read that as an attack on democrats and not as they calling COVID a hoax)
My initial reading was that he meant the severity of the virus is a hoax (which goes along with his "the flu kills more people" narrative).
Your reading sort of makes sense if "hoax" was a synonym for "attack". It's not, but Trump seems to use it as one, so yes I could see that could be what he meant.
It's interesting, because we end up in situations like this where you can try to read into it what he meant but that really requires guessing!
The Verge has got good, non-political coverage of the issue. Needless to say it isn't clear cut:
Trump surrogates said after the rally that he wasn’t calling coronavirus a hoax. “He was referring to the way he had been treated by the opposite party … in terms of taking every opportunity to bring him down,” said Surgeon General Jerome Adams on SiriusXM’s The Black Eagle with Joe Madison. And you can argue that Trump is limiting his claim to Democrats arguing he’s not prepared for the coronavirus. But the speech simply compares coronavirus to the “impeachment hoax” (which Trump describes as a “perfect conversation” that Democrats twisted into something negative), so it’s also easy to argue that Trump is saying the coronavirus itself is similarly overblown.
