This video is from six years ago and Molyneux has made the admirable shift from libertarian to HBD realism over that period of time, so I'm not sure he'd insinuate causation now.
In any case, there's a perfectly plausible, empirically sound mechanism to explain the fact that children who are spanked tend to have lower IQ than children who are not that has nothing to do with the corporal punishment itself. Intelligence is heritable, and less intelligent people are more likely to spank their kids than more intelligent people are.
The average IQ, as converted from wordsum scores assuming a mean American white IQ of 100 and a standard deviation of 15, of adults by how they feel about the statement that it is "sometimes necessary to discipline a child with a good, hard spanking" (n = 14,874):
Spanking doesn't lower intelligence, parents of relatively low intelligence--who, on average, have kids with relatively low intelligence--are just more likely to spank.
My guess is corporal punishment, so long as it doesn't result in serious injury, is about as consequential to life outcomes as other parenting behaviors are--that is, it doesn't make a difference one way or the other. It's long-term effects are as consequential as the decision of whether to play Baby Einstein or the local pop 40 station as you rock your child to sleep. It comes down to preference--I'll opt for Baby Einstein and timeout, but everyone's mileage will vary.
If it's primarily a cathartic exercise for the parent, that strikes (heh) me as being closer to child abuse than good parenting, but people work things out in different ways. It's not something my wife and I practice, but I see no reason to prohibit, say, blacks from doing it if they feel it necessary.
GSS variables used: WORDSUM, SPANKING, BORN(1)