Saying CASL applies to any message accessed in Canada is meaningless. Canadian law can't be applied to foreign entities. Reminds me of what happened with the national do-not-call list.
Of course it can. If they do business in Canada or maintain any assets their those transactions can be blocked or the assets frozen. Many developed countries have reciprocal agreements on things like court judgments, asset seizure and so forth. It's not the same as for a firm based in Canada, but it's not toothless either.