I think Microsoft at the time probably thought it's own engine was also better in many ways. It was in several regarding power efficiency and things like pen/pdf support.
Reason Chromium was chosen was more because they already had a bunch of developer experience with the engine.
That and more importantly it would cause Google to lose some control over Chrome and the web. Now Google can't exactly do further anti-competitor shenanigans like the whole shadowDOM fiasco and youtube. In this case adding random features to chrome, implementing them and causing other browsers to have a bad experience.
That and because edge is as fast as chrome (if not faster), there is less of an incentive to switch to chrome.
Oh and Microsoft is one of the few companies to be able to maintain a hostile fork of chrome and have it competitive. Largely because they have enough resources to match google for Chromium.
This is one of those cases where Microsoft can subtly claw back marketshare by playing the strength of chromium against Google. They wouldn't be able to do the same at all with Gecko/Firefox.
