alt Hacker NewsPresumably it's not, but the TTS voice in the video sounds to me more like formant synthesis than diphone - it reminds me of my DECtalk.
The project credits does mention espeak (which is formant based) as well as various other TTS projects, although it sounds like they are only using the pronunciation part of espeak, not the voice synthesis.
It certainly sounds similar, but seems more nimble with phonetic pronunciation in the demo.
Having it run on a pico would be pretty impressive =3
http://cmuflite.org/
https://github.com/festvox/flite