yes, it passes the smell test of plausibility; and it has two names attached; Sam Harris (whose reputation is either solid or iffy depending who you ask (iffy to me)) who then quoted Bertrand Russell, who has a solid reputation in a lot of areas.
Yet knowing the era he lived in, I know they believed some historical notions that later turned out to be false and yet are today still commonly believed, and I wondered if this was one of those things.