Yes, this is true. Unlike his friends, Mstislav Rostropovich and Galina Vishnevskaya, David Oistrakh was always a "dutiful citizen" of the USSR. I'm not sure whether Oistrakh was a true believer or just appreciated the perks of being "politically reliable" (or, perhaps most likely, some combination of both).
One certainty: Regardless of his political calculations, Oistrakh was undeniably a magnificent musician. And the same could be said of Ancerl.
All this discussion is making me think of David Remnick's book Lenin's Tomb and the film "The Lives of Others." Living in that sort of totalitarian state was . . . incredibly complicated. And nearly unimaginable for someone who hasn't lived through it (like me).