My impression is that Gates (and other tech billionaires) have been investing in fusion for a decade, so he probably knows or has leverage/access to people who know what’s going on, ergo Microsoft does too.

There is expertise in DOE and DOD, but there is an also a lot of private capital being invested in companies that don’t need to talk themselves up much. People in the industry generally know who most of the players are; that’s how you or your technical peers/friends get a job after all.

I don’t see a particular need for there to have been a top secret breakthrough to explain this deal.