Correct, while you can get a grey scale out of these films with low contrast developers, you will never get the exposure range of a Tri-X (or any other current general purpose film). It just ain't gonna happen. Even with Tech Pan, while I found TD-3 worked better than Technidol (a POTA variant) the total exposure range was still quite limited in comparison to general purpose films. Regarding POTA and similar Phenidone-based low contrast developers, it is tricky to use and is very prone to uneven development and streaking. One must remember when Levy formulated it, it was for photographing nuclear tests. The idea was to get as wide an exposure range as possible from film (not Tech Pan), and not much else mattered. Mottle, unevenness etc would not have been major concerns given the intended purpose.

A big reason for the popularity of Tech Pan, and why people were bent on trying to make it work with dilute Rodinal etc, was the notion propagated in magazine articles etc (I have several of them) that you could make prints from 35mm negatives look like medium format, and make prints from medium format negatives look like 4x5. That was based on the extremely fine grain and high potential resolution of the film. In practice I have never seen this work. I have an article from an issue of Photo Techniques (part of a survey of B&W films available at the time) that summarized the situation quite well.

This is not to say people haven't done interesting work with Tech Pan (and other such films), but the most success comes from playing to a given material's strengths rather than trying to make it behave like something entirely different.

Best,
Michael