Yes, but many of those wineries import their grapes from California (or other places).
My wife and I participate in this boycott and were drinking a sparkling white from Mission Hill Winery which is one of the biggest BC wineries. We noticed the label said something like "Produced in Canada" and then listed grapes imported from California.
Wines that are produced from 100% BC grapes are labelled "BC VQA" and only 19% of the wines in BC are BC VQA.