In the domain of hardware, what is the equivalent of a harness? How well would this generalise to any robot, versus one that is specifically built the way Xiaomi did?

Considering that they pre-trained on non-robot data, and mid-trained on data from other robot types? I suspect that generalization would be decent for most similar platforms - things with 1 to 2 arms and 2-finger hands.

You'd probably have to fine tune for good performance on a specific arm type. But this model isn't trained against hard assumptions of what the underlying hardware is.