Is this really an issue?

Technically, it's always been possible to do this with human programmers. I could read the code to Jellyfin, write out a detailed spec, hand that to a software engineer and have them recreate it. Or I could just come up with the same app myself from first principles. In most cases it's not really that big of a difference when you get down to it.

Arguably, that's what Emby did to Plex, or what Kodi did to MythTV. How much was inspiration and how much was copying? And does anyone actually care?

At the end of the day, patches and updates to the original won't work with your clean room implementation, so it's now on you to maintain this new codebase. And you still have to test it, work the bugs out, solve all the problems, and you can't just refer back to the original code for solutions because the whole point is that your code still needs to be meaningfully different. You haven't really removed any of the work of creating a piece of software. If you ended up borrowing certain details of implementation - some clever solutions and novel ideas - from your access to the nuts and bolts details of the original, that's just part of how open source works.

Clean room implementations are much more of a firmware issue than a software one.