Not only developing nations, Japan used bicycle infantry in their invasion of Indochina and Malaya to devastating effect - motorised vehicles were hard to use in the jungle terrain, but bicycles were fine while being drastically faster than the foot infantry used by British and other Allied troops.

Germany did that with Panzerfausts in the last days of the war. The riders were children.

Which is much better for the environment. (Y) Kudos

I mean... Ok. But they definitely aren't going quickly to protect from tanks. I'm not educated in the right ways to work out if shelling is worse than some bikes that'll probably outlast the war, if they don't get destroyed.

Depends on the tank and the situation - a bicycle from short range can be faster than the turret can be rotated to expose the bicycle to the machine gun of the tank. Not to mention the ambush opportunities in forests and the like, where the bicycle and the operators can lie down, be covered in netting, and spring up after the tank has passed them to hit it on the sides/back, and run away quickly after the attack.

Versatile mobility while keep the troops fit. Nice!