I dont disagree that the G is the 13th, all Im saying is, its not a 13th chord.
I tried to find some concrete info on 11th chords and their function, but nothing seems to explain this particular example. 11th chords are a sound that comes from jazz. They can be used as a 5 (voicing doesnt include 3rd) and then go back to the tonic. It can also function as the 5 chord that resolves to a dominant chord on the 5.
for example in C
the 5 would normally be G7 or G/GBDF or some kind of variation. Well one variation would be to turn that G7 into a G11 by playing the formula I said above with a G in the bass, and a F triad in the RH in root position. or G/FA
C. The C is what makes it an 11th chord. Notice this 11th has not been raised as in normal dominant chords. Its the natural 11 (4), which is a half step above the 3rd degree of that G7 chord. So if you were to resolve the chord like a sustained chord you could play G/FAC, then move to G/FAB before going on to the tonic or wherever.
Thats one function of the 11th, to use it as a 5 chord resolved or unresolved to go to tonic.
The function I was talking about though is when you use the 11th as a 1 chord or a substitution for a minor chord. Using it as a 1 chord, means it goes where a 1 would normally go, and it will give your progression an open/suspended type of feel to it. In jazz there are songs that are built off of this open type of voicing that I was talking about ie. Maiden Voyage, where pretty much the whole song is built on 11 ths (Im away from the chords right now but will update with the chords heard later)
So the voicing heard in the song is something similar to what I posted earlier
in C (away from the keyboard so I'll put correct key in later too)it would be:
C11 or
C/ BbDF (major triad one step down voicing)
another way to do it is to play a minor 7th chord built on the 5th degree of the chord or
C/ GBbDF (notice it still has BbDF in the chord which is what gives it that 11th sound.)
or you could voice it with a major 7th chord one step down or
C/ BbDFA (which is what we have in the example with that Isley tune.) When you have the major 7th one step down voicing it adds in the 13th degree of the chord (A), but it does not change the function which is still the 11th.
So if someone wanted to be real technical and call it a C11add13 that wouldnt be wrong, but it would not be a C13, because that symbol does not account for the 11th. Csus13 could also work as a symbol, but I think it would be a little misleading because someone reading suspended might believe it needs to resolve when in both songs Maiden Voyage and the Isley Bros tune, it clearly does not.
I hope that helps a bit.