That is a good thought. Apple has already explained their new chip. There is really no need to rehash the same type information with an hour video. The last presentation was entirely too long, in my opinion.
That is a potentially a press release if they aren't redesigning the iMacs and they're just swapping out the CPU/GPU/RAM with M1.
But the rumors are modest redesign with M1X silicon, which will be slightly different from M1. Apple would want to take the chance to talk about how much faster it is compared to previous Intel models and how it is improved since M1.
Non-snark question: Would Apple announce an M1 iMac if they didn't have the chips to satisfy initial demand?
Yes, they've done it before with iPhones. There was some mentions that Apple cut down on iPad production and strips them to maintain iPhone supply a few times in the past.
Also, keep in mind they released M1 Mac mini, MBP and MBA last fall with the chip shortage in place.
Another thing is, Apple pre-orders a massive share of their fab supply years in advance. TMSC chip shortages aren't affecting Apple as much as others, plus Apple's on 5nm process node that most folks aren't requiring yet for their designs.
On the other hand, if there's a redesign with new tech like mini leds or something, then there is a potential supply restraint from the new tech, not chip shortage.