I think it's pretty much a given in any guitar based genre that it needs to sound like a real guitar. There's also a difference between real and organic. Real guitars can sound heavily processed too. Doesn't have to be a real one either, some VSTs can sound great, and in the right hands you'd be hard pressed to tell the difference between real and VSTi.
What they don't want is obviously fake and stiff sounding facsimiles of live instruments - guitar or otherwise. No library would want it , as it wouldn't get licesensed - if you check out the Sports Rock section of any major library I would bet most of it is real, and if it isn't you can't tell.
I think the screener has given you the correct feedback here.
Mark