Halo Wars: Definitive Edition is a remastered version of the 2009 real-time strategy game that came included with the "Ultimate Edition" of Halo Wars 2. Two months after the release of the sequel, Halo Wars: Definitive Edition is the subject of two news tidbits -- one surprising, one not.
On April 20, Microsoft is releasing Halo Wars: Definitive Edition as a standalone product. This is the sensible end for something like this. First, use it to leverage additional sales of your new game. Once that well has run dry, try to wring some extra revenue out of it by selling it a la carte. (It's a small miracle that Activision has had the self-restraint to not yet offer Modern Warfare Remastered standalone.)
The surprising aspect is in the platforms that the Halo Wars remaster is coming to. It's obviously coming to Xbox One and Windows 10 PC, the two places it originally shipped for. But, it's also coming to Steam. Considering how intent Microsoft is on fostering the Windows 10/Xbox One gaming ecosystem, releasing a first-party title outside of that is kind of a big deal.
In the short-term, it's probably the best business move, though. More people will pony up for something like Halo Wars on a service like Steam. That's where it has the best chance to thrive. If Microsoft sees Halo Wars as anything beyond a two-game franchise, this is the way to raise the player count before making installment.
That's all dealing in hypotheticals from the relatively-distant future, of course. For now, Steam users will get a chance to see what the "best-selling console RTS of all-time" is all about.