Then again I understood exactly what it was saying every time, which is more than I can say for some of the other traffic on that recording. I’m not sure synthetic-sounding means bad here.

The embedded systems qualified for use in general aviation avionics have very limited hardware resources. They are severely constrained by form factor, power, and cooling. It's amazing that the developers were able to get speech synthesis working so well.

I could do better than this with pre-recorded samples for each word. Especially for the phonetic alphabet.

Also avionics aren't that underpowered these days. They have full touchscreen displays and multicore CPUs.