That might indeed be true. GitHub's value to Microsoft ain't the revenue; it's the users, the mindshare of whom Microsoft hopes to capture by luring them toward the Microsoft ecosystem (Windows, Office, Azure, SQL Server, etc.).

As a particular former Microsoft CEO put it: "[…] DEVELOPERS DEVELOPERS DEVELOPERS DEVELOPERS […]". That strategy/desire hasn't exactly stopped, though it's certainly transformed quite a bit over the last couple decades (and Microsoft is finally actually getting somewhat effective there).