He fell on the ground but didn't dq and then I lost the duel :/

Simply because he did not touched the ground.

If you look a little better, he extended his neck at the time that his head would disqualify him.

His chest red piece joint thing though..?

That my friend... seemed to be a glitch.

True, the torso touches the ground. But now I'm in doubt if the torso disqualifies, someone answer the poor guy.

the chestpieces or whatever they are only collide with the ground if they're dismembered

I can't watch replays (my toribash folder is a mess) but I imagine that there may have been the max contacts limit acting here. To prevent excessive lag the game only registers a certain number of collisions/contacts, you can change this in game rules. Therefore if you exceed this number of contacts by touching the opponent in weird enough ways, then the DQ will not be counted.

Most likely the max contacts glitching out.

I didn't realise that it actually glitches. Interesting.

Only joints dq. The torso is not a joint
- case closed