Of course he didn't speculate on impossible events. It is well known in philosophy that if you assume a counterfactual, i.e. something impossible like the earth ceasing to exist, you can prove anything including things which are false.
If the earth ceased to exist as matter, e.g. some unknown physcical processes converted it to photons",
then they would still result in the same gravitational potential field since teh source of gravity is mass-energy not just mass in general relativity.
There is a deeper question of energy conservation in general relativity due to the self-energy of the gravitational field itself, a completely different matter than gravitational potential energy which is the energy of objects in the field, which is poorly understood and does not appear to be conserved though this is controversial