This is fairly naive, Elon isn't the only investor in SpaceX.

My guess is "that they did the math" and had an engineering study which convinced them that getting AI datacenters into space will make sense.

It's also not hard to imagine why, the process alone once perfected could be reused for asteroid mining for example, then mirogravity manufacturing, either of which alone would be enormous capital intensive projects. Even if AI dataenters in space are break-even it would be a massive win for SpaceX and leave their competition far behind.