It’s all going to be open source, so basically anyone (including me theoretically) could port it to any app as long as it doesn’t directly compete with them and is compliant with their usage rules (which are ridiculously generous).
But (me personally) I don’t think I would do anything without getting their permission. This was a very lucky break for me and I would want to make sure they were OK with it.
Just speaking from my own experience though, Adesk hasn’t asked me for anything in H&F since about 2014, and actively fought me on any upgrades I wanted to do on it for several years prior.
Not speaking for Epic here, particularly since I’m not privvy to such things yet, but my theory is that they’re spending some of their gains on creating good will in the industry and attracting an ecosphere around their tools.
I’ve probably already said too much, but my feeling is their heart is in the right place and they’re really making an effort not to be bullies.