Apple could easily make the case that the platform was never designed for general computing. It was never an open platform, and what Epic is asking is for the government to 'compel speech' from Apple in the form of changes to their product in order to make it one.
If you steal an candy bar, the government can "compel speech" in the form of forcing you to return the stolen property.
This is a great example of a thing that is not remotely 'compelled speech.' Here's the wiki article on the topic[1]. There's a pretty high bar for the government to create exceptions to the First Amendment.
A better example of compelled speech is the FBI trying to require Apple to write software that would give police a backdoor into any iPhone user's phone[2].
[1] https://en.wikipedia.org/wiki/Compelled_speech#United_States
[2] https://www.eff.org/deeplinks/2016/03/what-we-talk-about-whe...