Frankly, I’m jiggered if I know!
My understanding of Mr Atkinson’s point is that it doesn’t matter. My understanding of his point is that the ‘squeeze’ caused by the interference fit is what holds the head on, rather than the strength of the threads.
If I get a bolt and clamp it in a vice and tighten the vice as far as I can make it go, it ain’t the threads on the bolt that stop me pulling that bolt out of the vice …..