Well, interestingly enough, Microsoft is also involved: and in spite of the fact they also own their own app store (the Microsoft store), they appear to actually be in Epic's corner.
People are saying this thanks to a new blog post made by Microsoft called "10 app store principles to promote choice, fairness and innovation", where Microsoft says the following:
"For software developers, app stores have become a critical gateway to some of the world’s most popular digital platforms. We and others have raised questions and, at times, expressed concerns about app stores on other digital platforms."
They don't overtly mention Epic Games or Apple in the blog post, but it seems like this is still aimed squarely in that direction when you read things like:
"We will not block an app from Windows based on a developer’s business model or how it delivers content and services, including whether content is installed on a device or streamed from the cloud.
Our app store will charge reasonable fees that reflect the competition we face from other app stores on Windows and will not force a developer to sell within its app anything it doesn’t want to sell.
Our app store will not prevent developers from communicating directly with their users through their apps for legitimate business purposes."
Microsoft being pro-Epic in this little kerfuffle probably won't amount to much. It's not as if they're involved in any official capacity, and since both parties don't want a jury involved, it's not even as if Microsoft can sway public opinion. But it's interesting nonetheless. And even a bit unexpected, in a good way.