mechanical disc brakes and bikes with internal cable routing

You seem to be conflating the housing and the cable in the last sentence.

Compressionless housing is generally better in my experience.

If your frame has internal, bare-cable routing through straight sections of the frame, there's nothing wrong with using in the sections outside of the frame. In fact, I would recommend it.

All mechanical disc brakes should have compressionless. The total pad travel distance is something like a fifth or a tenth of what it would be with a rim brake. Any length change of the housing is amplified in terms of waste lever travel by the same amount. Spyres and Spykes are good brakes, but using spiral housing on them can create problems that a caliper design can never solve, especially with routing schemes that require tight bends. Tight bends make the problem worse with spiral because an air gap is opened up between the coils at the bends, which then eats up lever movement before braking force is applied to the rotor.

If spiral housing is used, the power loss it creates and lever movement it wastes will be worse the more of it there is. And, as above, many full housing, internally routed bikes have some tight bends. That's what the ad copy is getting at. There's nothing wrong with using compressionless on interrupted housing setups.

Compressionless took/is taking far longer than it should have to become the norm for mechanicals, and it is a little finicky and has to be installed right, but many bikes need it for the caliper to feel good regardless of what model it is.