I guess if they already have a script, they’re past the WGA strike and can work on it till they hit the SAG strike. (I’m assuming the strikes impact Canadian productions on behalf of US studios?)
Yes, American productions in Canada have US writers and actors under WGA and SAG-AFTRA contracts.
Local Canadian actors belong to ACTRA, the Canadian guild. The production and art creatives are in another union IATSE.
DGC is a sister organization to the Directors Guild of America (DGA). Their accredited members can work in either country. DGC is likely now sanctioning the work since DGA has settled in the United States.