Canonizations are indeed infallible and require the assent of the faithful.
While the Church’s power to declare infallible doctrine is limited to the Deposit of Faith (i.e. those truths found either explicitly or implicitly in Sacred Scripture or Sacred Tradition) which ended with the death of the last apostle, there are certain dogmatic facts which pertain to historical matters after that point which if not held would contradict the Deposit of Faith. For instance, the Church can infallibly say whether Pope Martin V was a valid pope or not, for if such power did not rest with the Church, the entire doctrine of the Primacy and Succession of Peter would be rendered pointless. Likewise, certain men and women have been such extraordinary examples of heroic virtue that if anyone were to deny these people met with Heavenly reward and were in fact in Hell, then the Church’s very doctrine on Salvation would be undermined.
Thus, canonization is considered a dogmatic fact, that while the person might have lived after the Deposit of Faith, to claim that they are in Hell would implicitly deny at some level, the Church’s doctrine. This is the position historically held by most theologians.