It works, but is slow. I had 2,307 videos in my watch later... Not sure why it maxed out at that number, it stopped working at 1,000. Somehow it went higher.
Anyway it took about 30 minutes to delete all videos one-by-one. It REQUIRES that you leave your PC on the window, simply running a script over and over. So that's why it's slow.
Regardless it works :)
5/5. Being better is likely very, very difficult, though a selective do-not-delete list / selector would be an improvement no free extension has the feature (I have checked).