I was mistaken. It did delete the themes for me. I became confused because after clicking ‘Delete Themes’ after checking everything off, all I saw was the screen refresh and the list of themes was still there, so I assumed it was broken. However, I later realized it had in fact deleted them.
So I would recommend a notice popping up on the refreshed screen saying the themes were deleted. That would have helped me out!
But great plugin! It works as advertised.