What I found confusing was the name. "InvSqrt" reads to me as "Inverse square root"... Well, the inverse function of the square root function is just to square the number!
Apparently, what they meant was "1/sqrt", so ideally the function should have been called "ReciprocalSqrt" or something like that.
In fact, this is so confusing that I would argue that's actually just an incorrect name for what the function does.
It's true that mathematicians write "sin^-1" to mean the "inverse sin" function, for example, but they never mean 1/sin(x) when they do this, and they would never say "inverse sin" to mean "1/sin(x)".
Apparently, what they meant was "1/sqrt", so ideally the function should have been called "ReciprocalSqrt" or something like that.
In fact, this is so confusing that I would argue that's actually just an incorrect name for what the function does.
It's true that mathematicians write "sin^-1" to mean the "inverse sin" function, for example, but they never mean 1/sin(x) when they do this, and they would never say "inverse sin" to mean "1/sin(x)".