True or False?

is it true or not true in (your experiences) that the better your audio components are, the less they respond to various tweaks?

Not true. Just as a race car responds more to tweaks, so does a high resolving system. Part of the problem in tweaking a lesser system is that it doesn’t respond. Thus my skepticism in Power cords 15-20 yrs ago or so. Tried them. Nothing there. 10 yrs ago, aha. They did work along with many other tweaks I dismissed i.e. Oyaide R1 outlet. Then too, I had developed more listening skills which is essential.

I concur with Artemis-5. I went through the same progression that he did. As my system got better and more resolving my listening skills also improved . My appreciation for music was always there but as I continually upgraded my system;I heard more details that were obscured by my earlier systems.

The better they are the less tweaks needed to make them sound good.  See how that works?

False, the more resolving and balanced the system is the more sensitive to tweaks whatever that would include, cabling, room treatment, fuses, power distributors, ac receptacles, vibration control.
As for manufacturers, they are not  denying anything for a simple reason that all use better cabling and better means of support to evaluate or demo.