I've always felt it should be possible to get a good seal without
bodging with silicone or plumber's mate, especially if the surfaces are
clean. A bit of washing up liquid for lubricant, that's all.

This guy uses one:

https://www.youtube.com/watch?v=jaPsf2xTZfc

but also uses silicone, which is cheating IMO

I agree with your thinking - but in this case the thread itself is the
leakage route.