To make a valid comparison of the benefit of different wheel diameters, all other parameters must remain the same: same bike, same type of tire. Comparing different bikes equipped with different type of tires doesn't allow to know what bring the difference, the wheel diameter or the bike or the tires.
The test with the Birdy isn't interesting because its a Birdy but because this bike accept easily several wheel diameters including the very frequently used for folding bikes ETRTO406..

I have a Brompton and a Birdy and the Birdy is much more stable/less twitchy than the Brompton even with the same ETRTO349 wheel size !

Note also that the overall wheel diameter relative difference between 50x305 and 40x406, even if it is not huge, is bigger, about 20%.