The result can only be distributed under the terms of the GPL-3. That's actually a crucial difference: there's nothing preventing Kitten TTS from being Apache licensed, soliciting technical contributions under that license, and parts of its code being re-used in other software under that license. Yes, for the time being, this limits what you can do with Kitten TTS if you want to use the software as a whole (e.g. by embedding it into your product), but the license itself is still Apache and that can have value.
The result can only be distributed under the terms of the GPL-3. That's actually a crucial difference: there's nothing preventing Kitten TTS from being Apache licensed, soliciting technical contributions under that license, and parts of its code being re-used in other software under that license. Yes, for the time being, this limits what you can do with Kitten TTS if you want to use the software as a whole (e.g. by embedding it into your product), but the license itself is still Apache and that can have value.