" issues around Google's actual clean-room implementation are largely irrelevant at this point in the lawsuit"
Are they? I am not an expert on this, but isn't the fact of actual copying (rather than successful clean-room implementation) a very relevant piece of information for copyright?
Actual copying is important, but not in this particular lawsuit.
Oracle is claiming that the organization of constants/methods in packages as well as the method signatures are copyrighted - they are arguing that it doesn't matter if you have a clean room implementation of BufferedInputReader() if you put it in java.io (like Oracle's JRE) and gave it the same signature as Oracle did, then you're infringing on their copyright.
If this SCOTUS broadly rules in Oracles favor, this would kill many projects, including WINE (which implements Microsoft's Win32 API without copying)
"At this point" is key, actual copying was important earlier in this case but is now a decided issue.
This appeal is strictly about the copyrightability of APIs. The supreme court generally doesn't address cases as a whole, but specific issues brought up to it on appeal.
If I remember the decisions on the actual copying part of the case correctly, it was decided that Google copied something like a dozen lines of uninteresting code directly (not api's, just some method implementation), and that this was de minimis copying and didn't constitute infringement. You should check if you really care though, I might be wrong.
Are they? I am not an expert on this, but isn't the fact of actual copying (rather than successful clean-room implementation) a very relevant piece of information for copyright?