I would posit that the assertion is true
provided that xGA is being measured properly. Spoiler: it's almost certainly not, unless someone is measuring xGA not just on the (x,y) location of the shot on the ice surface, but is factoring in
at least the following:
* The velocity v of the shot
* The angle (α, θ) of the shot - specifically, the (y,z) spot of the net the puck is shot toward
* The direction γ, distance d and velocity v' of any pass to the shooter, given the (x,y) location of the pass
* The length of time t' the shooter handles the puck before shooting
* Whether the puck is deflected in any way, where (x,y,z) the puck is deflected, and the resulting new trajectory (v, α, θ) of the puck after deflection
* The goalie's (x,y) position on the ice when the puck is shot
* The goalie's specific stance (upright, butterfly, double-pad stack, etc.)
If
all of those are being measured and have an xGA that can be calculated given all of those measurements,
maybe I might buy the argument. But that still requires actual GA relative to xGA to move toward its goalie-specific ratio over some time
t, and for that ratio to not change based on anything else.
See where I'm going with this? It requires
a metric f***ton of data to get an accurate calculation of xGA, and then the assumption that
all other variables that impact a game's outcome have zero impact on GA relative to xGA. Oh, and it also implies that said ratio is the same whether skaters are on the ice or not, or where they are on the ice. [Unless you're factoring that into your xGA, which ... if you're going to incorporate all the stuff I list above, you should probably incorporate that, too. Cause, I didn't mention if the goalie is screened, or has been bumped at some point prior to the shot, or other things that impact a goalie's ability to focus on a shot / see it to be able to stop it. A few other things too, which I'll leave to everyone else to figure out. I'm not doing all your deep thinking for free.]
Short: I think it's a claim that might stand up, provided one can measure xGA at a sufficiently granular level to account for all possible variables that impact the probability
p of any shot resulting in a goal. I think all
teh analytics folks are
a long, long, long way away from accomplishing that, though. In the current world, given how xGA is by and large getting measured? No way in hell is that claim remotely correct.