From someone who has a limited reverse speech vocabulary, that was impressive.
When I was in community college radio, using an open reel tape deck, I learned to speak a few phrases in reverse well enough to be intelligible when recorded and then played backward. I have software to allow this on my PC now, although I have kind of lost the fascination with doing it.

Listening to the reversed sound is just about the only way to do it. Obviously, spelling the words backward and reading them doesn't work. This guy has too much time on his hands. Still, an impressive feat.
No matter how I tried, there was always a "reverse speech accent", and this guy has it too.