Cuba might get the short end of this one. If it becomes too embarrassing for our military/government they might cut off relations. And Canadian tourism is a big sustainer of Cuban coffers. Canada could cut off that income by blocking travel there, or even just declaring it as an "avoid all travel" zone, effectively cutting off tourism.
The truth is, we don't know what's going on. Maybe this was a deliberate decision to get closer to the Russians without triggering an incident. Maybe the US knew about this and approved it. Maybe it was a blunder, but we can't just assume it was.
As the elected Prime Minister, you get automatic top secret security clearance. The expectation is that the democratic process would have put the person in power, so we want that person to do whatever they wish with that top secret information. Otherwise, we would be losing the power of the people and putting it in the hands of CSIS.
All appointed ministers do get security screening, as that was put in place by Harper, I think. As a leader of the opposition, Poilievre wouldn't be subject to those requirements.
As far as I know there's nothing to indicate that he wouldn't pass the check. He just doesn't want to get the security check and get the inside info, because that would limit what attacks he could make. As an outsider he can spout off whatever drivel he wants.
So, if he doesn't pass your security screening, citizen, get out and vote.