Andy Murray is
not the 13th greatest tennis player to ever play the game..
A couple things. One, Ultimate Tennis Statistics is not about "to every play the game" - it only uses stats from the Open Era.
Two, it is just a statistical metric. It is not necessarily saying that he is the 13th greatest player of the Open Era, just that he's 13th in GOAT points, which are based on statistics that--as you and others have said--really only make sense for the last 25-30 years (that is, since AO has become comparable and the Slams have been so esteemed).
That doesn't mean the GOAT points system doesn't have value, it just shouldn't be taken as absolute. Actually, it is a pretty nifty formula, that takes into account all that you could ask of such a formula, and does so in a balanced way.
One thing it does tell us about Murray is that he is better than his "mere" 3 Slams indicate. This is illustrated by the fact that he's reached 11 Slam finals - the same as Edberg, McEnroe, and Wilander. I am not NOT saying he was as great as those three, but it does tell us how often he was in the mix for a Slam title. Unfortunately for him, he played alongside three 20-Slam winners.
Anyhow, if you consider Open Era records only, how many players ranked below Murray in GOAT points are clearly considered to have greater careers? I'd say Wilander is the only clear-cut case, with his 7 Slams. But the rest of the top 20--Vilas, Nastase, Rosewall, Ashe, Newcombe, Hewitt--all had inferior
Open Era accomplishments (most of Rosewall's best years were before the Open Era; Newcombe's career as split, etc).
So if I were ranking players of the Open Era, I'd probably have Murray at #14, just pushing Wilander higher. I'd say he's pretty comparable to Vilas and Nastase (who was also better than his Slam count). If we expand to all time, then he's probably not in the top 30. Off the top of my head, Tilden, Perry, Budge, Riggs, Kramer, Pancho, Rosewall, Newcombe, Cochet, Vines, Trabert, Sedgman, Wilding, Renshaw, Doherty, and probably a bunch of others.
I mean, if we really want to give
all of tennis history weight, we have to remember that there were over 90 years of tennis before the Open Era.