I think he intends for Tesla to be the end use for all the cells. Unless their products start tanking, in which case he'll pivot.
I could see them supplying cells for stationary energy storage if Tesla energy is at capacity, but I suspect they would only become a cell supplier to another auto manufacturer if that manufacturer was serious about EVs and willing to commit to a long term contract.
That must be what he’s thinking. He’s said he sees a need for 100 gigafactories to get us onto sustainable energy.