The cost at such they could rent out the TPUs, i.e. the market rate, is the inference cost.
Just because you are vertically integrated doesn't mean you get to discount the one business units products to the other. Doing so discounts the opportunity cost you pay and is just bad accounting.
Basic business principle, you charge what people are willing to pay not what it costs.
Look up “double marginalisation”.
Depends on if you have spare capacity I think. They have minimal competition so they might be maximizing profit by charging prices higher than what clears all their supply.