You'd need to keep one side of the thermocouple cold in order for it to work, and now we're back to the problem of dissipating heat...
If you boil water, now you have steam that you either need to vent overboard (limiting the lifetime of the system), or recondense, necessitating the dissipation of heat...
Setting aside the environments arguments, I'll hold out hope that he's wrong and none of the ML companies will ever be able to fund something like that.
Also, what a weird launch company to buy if that's your goal. I thought Relativity was going to be the odd one out without enough differentiation and wasn't long for this world. The data center rationale makes me question that decision even more. I'm not praying for their downfall or anything, I just don't get it.