Even they will tell you that 75% of gm vehicles are now built in mexico and canada in non union plants.
Do you believe everything you are told? Most GM vehicles sold in the US are manufactured in the US. Not all, but >50%. I don't have a handle on Mexico, but would expect, and a quick Google indicates all or close to all of their facilities in Canada are union shops. GM has the highest percentage of US content in its US made vehicles. GM sell AND makes more cars in China that the US now. For clarity, they are sold and made there.
Passenger car MFG is moving out of the US and Canada because the sales are not there, and overall costs are lowest near your customers. Mexico can ownership is growing. Pickups and SUVs, are where the sales are in the US, and there is still strong MFG in the USA of those.
Another point, is it is best to look at dollars, not total numbers. It is lower cost vehicles being shifted into production in the Mexico and other lower cost centers, higher dollar vehicles not so much.