Back before cookies existed, it was common to use arrows and clothespins to cancel markers or mark t's. I was taught this by my cave instructor back in '95.
Obviously, opinions have changed drastically but as recently as 5 years ago there were one or two instructors that still taught this. It's quite possible OP's buddies were taught cave diving years ago.
This here, how can training agencies and instructors reconcile those two completely different approaches? It is then left to us mere divers to try and work it out so that we can dive safely together. It shouldnt be my job to convince diver #1 much more experienced than me who was taught „arrows” that this is not the way - he wont listen as I am younger and less experienced. Diver #2 was trained more recently but does „arrows” too - he was trained in same area as me, but with a different centre - how is this possible to have two drastically different curricula within the same instructors’ community?
I was not explained during my training why cookies and not arrows, but my argument to buddies #1 and #2 was that it is quicker and easier to recognize a cookie rather than notched arrow in an out of viz stressfull situation. Manatee Diver gave me another argument, as theoretically when marking a T with an arrow you can place your arrow pointing to other than nearest exit permanent arrows in the vicinity.