This seems like some kind of logical short circuit? An army without any armour at all can still have heaps of artillery, drones, planes, missiles, etc.

Russia running low on armour means exactly that: They're running low on armour.

There's plenty of evidence suggesting they're running low on other things as well, but that's not the point being made here. The shortage of armour is especially significant because it means russian assaults become less effective and more costly (read: more dead russians per assault).