idk I always assumed they never called thought-speech telepathy because it's not really telepathy
I mean throughout the series, there are three or four DIFFERENT kinds of telepathy which would get kind of confusing if she had simply referred to them all as "telepathy." There's the way the Ellimist communicates, the way the Leerans communicate, and then the Andalite version, each of which are nonverbal and slightly different.
If anything, I wish she would have been more specific with thought-speech. It's telepathy, but everyone seems to have a "voice," at least, people can tell who is saying what. You could just say this is everyone's *personality* shining through, but the few times it happens, there never seems to be a problem when one Andalite morphs another. Didn't Aldrea sneak into the room with the Quantum Virus by morphing Alloran? Wouldn't the guards have been able to tell that her voice was all high and feminine?
And then at one point in The Andalite Chronicles Elfangor say something like, "that's not really how thought speech works" when referring to specific words, WELL THEN WHY THE HELL ARE THERE SO MANY MADE UP ANDALITE *WORDS* ugh idgi tbh