I don't have any real numbers to back it up, but I have been told that Red Hat costs the government more money per machine for the licenses. At least within the DoD. Now this could well be attributed to buying less licenses, and not getting as much bulk discount. But Red Hat not necessarily a cheaper option. And even if you got the license costs down you would still have to pay absurd amounts in retraining the entire workforce.

DISA is a big group. They have mountains of RedHat instances already as well. I would bet they've probably got a billion dollars or so invested in the RedHat infrastructure.

If I had to hazard a guess without seeing the SOW/PWS, I would bet that most of this MS contract would cover desktop support services.

DISA supports IT services for a huge portion of the DoD. This is basically a Fortune 100 company in terms of employees and scale.