I don't know how true that is about YouTube intentionally CPU bombing ad block users specifically.
I will say though that websites run by big tech companies getting much heavier and more poorly optimized is ultimately just a fact of life. They don't care about optimization, in fact it benefits them more that people have the latest and greatest hardware so that's what they're going to target. Ultimately that means that these websites will get slower on older hardware with time, and people will rely more and more on alternative frontends to access content, and it won't really matter if you have ad blockers or not (*turning it off on a bloated site that is bogging down your CPU might actually make it worse).