It sounds like this one is the case, if you look at the VG247 article.
As for other SAG-AFTRA actors doing voice work on the game, I checked the SAG rules and the relevant one states:
No member shall render any services or make an agreement to perform services for any employer who has not executed a basic minimum agreement with the Union, which is in full force and effect, in any jurisdiction in which there is a SAG-AFTRA national collective bargaining agreement in place. This provision applies worldwide.
So I guess it hangs on what the agreement that SAG has with Gearbox entails. It could be that Baker wanted more union actors that the agreement requires.
I guess he could also have a personal beef with Randy Pitchford and the union thing is just a cover, but I feel like if that were the case he would be vaguer about it instead of saying that Gearbox wouldn’t go union.