It's actually not as tricky as it looks, especially when the microphone is plugged into a "valve screamer" - when the guitarist plays, the sound is less distorted than the singing! It's only like being a "singing dalek" - use a bit of distortion and delay on the voice. There are much better clips on Youtube of "mouth guitar" ... keep looking.
When I was young and played with a jazz band, I used to do mouth "clarinet" and "trumpet" solos (can't do clarinet these days, that is very hard).
LOL you young fellers should be listening to records of the Mills Brothers or the Flying Pickets for good mouth music ... Don't Worry, Be Happy (Google that, it's a song).