I could think of a number of hypothesis off hand.

One is just that from a thermodynamics point of view there could be some utility in having data processing facilities floating in a heat sink.

Another would be that Google themselves feels some benefit in having the potential to operate with some flexibility in terms of jurisdiction and physical security.

Another is that they have so much money they just don't know what to do with it.

Another is basically what the author of this vid proposes.

One way or another, it would be an interesting thing to watch if there is anything to it at all.