Assassination is not an invasion. Israel has been assassinating people for decades. Should we take their nukes away too?
You're right. Iran trying to assassinate a Canadian MP is not the same as an invasion. But we were talking about the threat Iran (and their potential nuclear weapons) pose to Canada. Even if you want to discount the possibility of one of those assassins sneaking a nuke into Canada, Iran having a nuke while hostile to Canada is a problem.
After an assassination attempt, Canada would put political pressure on Iran but those options would be limited if a nuclear Iran (backed by Russia) can just tell us and the world to get fucked.