Partially answering my own question with some basic googling: apparently, under modern Canadian law, revocation of allegiance is not the same as revocation of citizenship; you can only renounce Canadian citizenship through a special application process.

s/revocation/renunciation/ pls in the above.

I wonder whether revocation of allegiance to a prior government (as part of the US naturalization process) is equivalent to revocation of citizenship. If so, that might exclude a large number of Canadian ancestors who migrated to the US (including a couple of mine, possibly).