“Biden conspired with Dominion to tamper with votes” is a statement which can be assessed factually, and which has significant impact. Carlson and others knowingly lied like that – hence the first massive lawsuit settlement – and the J6 insurgents show every sign of having believed those lies and being motivated by them to assault the legitimate government trying to prevent a fair election result.
The First Amendment is not absolute and has long-standing exceptions. One of those are false statements about specific people, which Carlson clearly made knowing they were false before he made them, and another is incitement to violence. The bar for that is high but it seems likely that at least some of these calls are debatably close to that level.
It’s also worth remembering that the first amendment isn’t relevant to what a Canadian regulator allows to be broadcast in Canada.