Is that Microsoft Sam? :)
(Also, I know it's besides the point but this might be the most painful way to connect to Wifi physically possible. "Make normal everyday tasks slow, tedious and painful" is a bit of an odd choice for a product demo.)
Say, speaking of Sam, what were the memory requirements for SAM (Software Automatic Mouth) on C64. I guess they were not more than 64K? Although, the bulk here is probably for the speech recognition, not the TTS. (And this one does sound a little nicer :)
Browser demo of a reversed SAM:
There are two Sams here, the Microsoft one and the C64 one. I don't believe there's any connection between the two other than the name.
According to [1], the weight of a modern runnable version is around 39k.
The ratio of how good it sounds compared to how much computing power it uses is ridiculous. The C64 has ballpark 3 orders of magnitude less CPU throughput as an RP2350, and the codebase uses an impressive array of tricks to do actual formant synthesis (barely) and a pretty refined form of Elovitz text to phoneme conversion. One of my favorite tricks is its up and down bouncy pitch, which is not random, but based on the opposite contour as the first formant. It's simplistic but enough to make it not sound like a robotic monotone.
I've been playing around with this some myself and SAM is an inspiration, along with other landmark systems like MITalk (predecessor to DECtalk), SP0256, and other. I believe it's possible to use modern techniques to get pretty good sounding speech in, say, 64k and 10% of the throughput of a RP2350. It's really cool to see projects like OP, especially under permissive license.
[1]: https://simulationcorner.net/index.php?page=sam