I don't think the original authors mind others using the word "YOLO". It's just ass-holish to call it YOLOv5 if you're not the original author, if at the very least because the original author is probably already working on something they plan to release eventually as "YOLOv5".
If they had called it FOO-YOLO, YOLO-BLAH, YOLO++, or literally anything else, it would probably be perfectly fine.
The original YOLO author publicly announced that he was no longer going to be working on computer vision models and has chimed in and said he has no problem with the name: https://twitter.com/pjreddie/status/1272618558254534657
If they had called it FOO-YOLO, YOLO-BLAH, YOLO++, or literally anything else, it would probably be perfectly fine.