There's no legal mechanism for that either. This is all just wild "what if magic unicorns showed up and started stabbing everyone" speculation. The only way Canadian citizenship can be revoked is if you committed fraud when getting it in the first place, Canada would have to pass new laws specifically for this purpose.

Technically, the Citizenship Act is a regular act of parliament and could be revised by a similar act of parliament. But not really. There's a bunch of Charter rights involved - mobility rights, security of person, equality rights - so an act like this would instantly run into numerous constitutional challenges.

There's also precedent. During the 1995 Quebec referendum, the federal government's official stance (and the consensus among constitutional lawyers) was that even if Quebec voted to separate, Quebecers would not automatically lose their Canadian citizenship. Because citizenship is an individual right granted by federal law, the separation of a territory doesn't magically dissolve the individual legal status of the people living there.

Frankly, spending even this much brainpower thinking about this matter is a victory for the separatists. Their whole thing is idiotic lunacy from the ground up and the only thing worth spending energy on is "how can we smack them down even harder" rather than "what if they somehow succeeded?"