it's a KASU
I underlined it and drew all sorts of nasty "THAT MAKES NO SENSE" notes all over it.
"And since I was in a human body, I too was restricted to spoken language" 27, Book 8
Then in Book 33 he relays all of what's happening at Tom's coronation or w/e the hell that was to Tobias, THROUGH THOUGHT SPEECH IN HUMAN MORPH.
Thought speech is one of the most inconsistent things in the series. The spatial limits applied to it change according to the requirements of the situation. In one book, they're all flying up to a mile apart and joking about fried eagle drumsticks or whatever, and then in The Underground Rachel can't even get Ax's or Tobias' attention 25 feet below the surface of the ground. And you could provide all sorts of fanwanky explanations, like "MAYBE IT DEPENDS ON THE SIZE OF THE MORPH" or "MAYBE ALL THE DIRT ACTED AS A FILTER" or "MAYBE AX JUST DIDN'T KNOW HE COULD THOUGHT SPEAK" but you know what? When it comes down to it, it's just inconsistent.
And whatever, tbh. It would have been nice for there to have been some rules at the beginning. At the very least, any morph--even if they morph from their human forms into another human body--should have the ability to TS. The technology should not discriminate between cranial functions or motor abilities or verbal capacity or anything. If they can TS as dolphins and, like you said, Hork-Bajir, they should be able to TS as humans. I feel like if you really push it, though, using the Escafil Device should have downloaded the "Thought Speak Version 1.0" program into their brain in the first place. I mean, it would have been a weird design decision to add that, since every Andalite could already TS and they were the only ones the technology was intended for, but unless there was something in the programming that allowed it, they shouldn't have been able to communicate in morph at all. In that case, EVERYONE should have been able to TS in their natural, human, hawk, or Andalite forms, once they've touched the legendary cube. But of course, this would have mollified a lot of really great, tense moments throughout the series, soooo I mean in the end, it's the decision you have to make with every KASU: was the inconsistency worth the story telling capital it gained?
I think, in this case, yes, but I will admit that on this reread of the series, it's the thing I'm having the biggest problem with.