I have no experience doing "Dirt" biking down hill or in muddy country tracks off road? But if we find somebody who have ride this bike they could tell if the very small wheels makes it difficult.
Look here.
http://www.clou.at/clou_datenuk.html
Technical Data:
Handlebar: Height 74 to 91 cm
Seat: 55 to 93 cm
Brakes: Rear and front drum brake
Folding pedals
Internal 3 or 5 gear hub with grip shifter
Tire: 3.00x4" with inner tube
Now these wheels are wide but small mini scooter wheels? Maybe Italian Vespa? How does one translate 3.00x4" to ordinary wheel size? somewhere between 12 and 16" maybe.
The guy who made this folding bike make them in for hand. Welding them together at a workshop as far as I know. He most likely makes "stunts" on them too. I ve seen pics on it. So that could explain why he didn't use ordinary wheels. It is a special made bike for his needs as I get it.
the folding tech is very similar to the Mobiky way of folding. Try to see any differences and tell me. Both of them are pat pend so that is kind of surprising. I mean what is different about the way they fold?
ok back to wheels. He use Scooter wheels cause that is the only thing strong enough to allow the jumping down from stones to sharp edgy lower stones of road? Or when they do the stunts they jump up and down with the bike all the time.