The idea with true temperament frets is that each string will intonate as well as possible for each note along the length of the board. Theoretically, if notes are combined into chords whatever the key they will be in tune far more than standard frets regardless of key.
Also in western music, as you know we have twelve notes per octave. Each pitch is an average for each key (well or equal temperament), as mathematically the different keys would have slight differences between notes, e.g. a G# would not be exactly the same as an Ab ("true" temperament).
So this would lead us to most tuning being a compromise on most instruments using equal temperament.
Back to frets, the true temperament frets are a bit of a misnomer as although they intend to provide better tuning for each string at each fret it has to be back to an average for each note and it's enharmonic so it is not totally true in terms of temperament. Even if it could be you would still be slightly out of tune with a keyboard using an equal or well-tempered tuning.
So all tuning is a compromise and especially so on a fretted instrument.