according to Tom Scott's very informative summary of all things youtube and copyright [1] there are seemingly reasonable alternatives to suddenly shutting down a whole channel - it seems like we're not getting the whole story as to why those were rejected or aren't being considered in this case?
Remember YouTube and its staff will never make decisions on the validity of copyright claims. They put their safe harbor position at risk if they do so. Instead they consider every copyright claim as valid unless legal proceedings tell them otherwise.
> They put their safe harbor position at risk if they do so.
Only if they don't comply with a DMCA takedown notice, which allows the supposed violator to claim responsibility. That is, whoever posted the video can say to Youtube "This video doesn't infringe on anyone's copyright, let them sue me if they disagree" and Youtube would be in the clear according to the DMCA.
From what we can see in this case, Youtube didn't receive a DMCA takedown notice, since they didn't make it available for the channel owner. Youtube has other internal processes (like Content ID[1]) to detect and remove copyright content, it looks like one of these processes is what triggered the copyright strikes.
[1] https://www.youtube.com/watch?v=1Jwo5qc78QU