Earlier quoted context omitted.
While it's perfectly legal for your wife, a Canadian, to go to Cuba, it's still embargoed by the U.S., and Slack as a U.S. company must comply with the embargo (even though your wife has done nothing wrong per Canadian law).
That's a good point except that Slack shouldn't be banning people that are not violating sanctions. In this case, if the account was actually opened from Cuba ... that could be a problem. What they should do is offer recourse and a way to have this resolved.
So they are absolutely offering recourse and resolution, but only where it is in their power to do so.
TLDR: Don’t expect Slack to be responsive to arguments that contain “please ignore US law for my individual circumstances”.