Epic is the developer of the Unreal Engine, a very popular game engine used several developers in mobile and non-mobile realms. Epic is currently suing Apple over the latter’s ban on Fortnite and Apple in retaliation has threatened to revoke its support for the Unreal Engine on iOS and MacOS.

This is part of its attempt to cut off Epic from using Apple’s development tools, for which Epic is seeking an emergency injunction. Apple later on released a sharp response to this injunction, stating that Epic had brought “this on itself” by starting the whole thing. Epic on the other hand, also responded with a new court filing also stating that it’s not asking Apple to play nice like nothing is wrong.

So! You might ask, how does Microsoft come into all these? Well, here’s how: Today, Microsoft filed a statement, which was clearly in support of Epic. This statement was written by Kevin Gammill, the General Manager of Xbox’s third-party developers.

He stated that the Unreal engine is too valuable a piece of technology for Apple to effectively cut off. The statement made was due to its importance in helping developers in providing a virtual environment to build their games in.

Quoting Kevin Gammill’s words, “Denying Epic access to Apple’s SDK and other development tools will prevent Epic from supporting Unreal Engine on iOS and macOS, and will place Unreal Engine and those game creators that have built, are building, and may build games on it at a substantial disadvantage.”

With the way this battle is going, Microsoft is going to benefit largely if Epic cuts Apple off at the knees, allowing users move to Microsoft’s xCloud. Microsoft has lots of games on the iOS platform running on the Unreal Engine, so having a fallout and eviction by apple is tagged to be a fight over real money, due to the number of users that are currently fed-up with Apple’s continuous bans, e.g. Fortnite.

Source:
https://www.smartphonecamp.com/microsoft-sides-with-epic-in-war-against-apple/

That statement doesn't imply taking sides. Microsoft only stated the implications if Apple goes ahead to block Epic products on its app space (which is what they can do and not deny them the SDK as the article implies. These SDK's aren't licensed so anyone can have it).