Is RuneApps/Alt1 Toolkit safe and legal?

Okay, so while trying to find out how to solve a Celtic knot, I stumbled over a few older reddit posts regarding this here toolkit. What it is, essentially, is an overlay that doesn't touch the client (from what I know) and acts sort of like the overlay app for Team Speak. I used the overlay to help me solve a Celtic knot, which I had to bring up the overlay and the Celtic knot, which it then took a picture and showed me the steps to solve it. I did all off the clicking, it didn't interact with my game at all.

I really just wanted to come to this subreddit and ask, hopefully getting confirmation from a Mod and if it is legal and safe, to promote this wonderful tool. Thank you for reading.