Why don't you use your distribution's packages manager to install it?
Gimp is actually overfeatured for what they need, but that was the only thing I found which I could install with a good enough certainty that I won't infect their computer with malware.
EDIT: I mean drawing geometrical stuff. Of course it's good for general drawing.
They are knowingly using this dark pattern to make ad money which is too bad, because as you say the app is really good.
I just remembered that there's also Dia (diagram editor), I might give that one a try.