Maybe it is worth looking at the tread-title and the OP's question again. There's probably no doubt that "classic MTB-sizes" have clear advantages in some situations off-road. I thinks the OP knows this, too. But he explicitely asked for the "best off road 20" folder".
In my eyes this still is the Airnimal Rhino (at least of the bikes that I know). BFs are pretty robust but lack suspension (which is a huge tradeoff offroad). Plus the long stem porbably is a disadvantage when seriously off roading. Moultons may be able to do a fair bit of off roading but they are clearly not built for that purpose and it would be mistreatment. Plus the suspension is not designed for off-road use (far too little travel). The Jetstream may be able to do the job to a degree but with the risk of damage. Plus Dahon themselves say it is a road bike that should not be used off road. And the Downtube (the only one in the list I never saw) looks like a proper bike, but also with limited abilities (or on-purpose-design for off roading) in comparison to the Airnimal. Plus it has a hinge on the main frame which for me personally would be a bit of an issue for an off roader. The Airnimal is built like a tank, well a small tank - in fact like a higher level MTB, just with smaller wheels. Therefor I'd trust it more offroad than the others. No shame on the Downtube though - just by looking at the different price level of the two bikes you can easily see where parts of the difference may come from.

Regarding 20" off road in general one should keep in mind that there is BMX and that there is bike trial. Both done with 20"ers. So, depending on the terrain, 20" may be a disadvantage or even an advantage. No doubt there are situations where the rider needs to have more skills than with a bigger wheeled bike - but I think the OP is aware of this. He doesn't look for the best off roader but for the best 20" off roader.