I believe I heard a radio interview on this story a few days ago, before the results of the vote were revealed. If I remember correctly, they said that only scripts written by Canadians would be directly affected. Any shows that have American writers but film in Canada (and there are a lot of those) would not have to stop.
Which changes the complexion of the walkout threat entirely.
Canadian production companies that are signatory to the WGC Agreement would be without writers, American Studios are signatory to the WGA so, if there was to be a strike, it would be Canadian content that would suffer.