They can vote “in theory”, but probably not. U.S. Citizens of Puerto Rico, the US Vigin Islands, Guam, and other territories do not vote. Only citizens resident in the States and DC can vote. So, an overseas citizen would either have to maintain a residence or have recent ties to a State. I think a lot of these overseas citizens have their citizenship by accident or because their parents were citizens, but have not lived in the US. They should not be able to vote.
It’s not that they ought not be able to vote, it’s that they literally can’t if there is no state in which they’re resident. There is no popular vote, there is no place to count these votes outside of state elections.