It's easy to see the benefit in DC's in space if you look at a few ingredients:
1. The recent Iran drone attacks on AWS data centers
2. Growing anti-AI and anti data center sentiment at home, plus Larry Fink (ceo of Blackrock) in a recent interview being terrified of dissident groups using consumer drones to attack data centers.
3. Anthropic, Grok, and other AI vendors becoming more and more integrated into defense and military, plus increasingly reliance on AI for other national surveillance systems
Data centers are and will be targets, both for national military attacks as well as home grown dissident attacks, so they are proposing to move some of the critical workloads to somewhere that the only group that can attack the data center hosting the workload is a nation state with space launch capabilities. That significantly reduces the number of actors that can attack the data center. And if the US wants to they can probably bomb all the other space rocket launch facilities worldwide in less than 24 hours, leaving extremely limited capability to attack a space hosted DC.
Is it insane? Probably, but the US has done insane things with military budget before, and will continue to do so for a long time. If you are Elon, its a great time to milk that US defense budget for some more R&D, and even if the main project doesn't work out, he's still going to be able to keep some innovations within the company and apply them to Starlink and other more realistic endeavors.