I do know the grasses need to be actively growing, but we routinely spray grass in our Christmas tree rows that is 2' tall and it kills it. I would guess that it varies from species to species to some extent and possibly annual vs well established perennial types.
I just looked over the publication you mentioned, thanks for that link it is a very well written publication and I will save that one to my files for future reference. It does reference the fact that mature perennial grasses are harder to control with Cleth and may require multiple applications. I can see that especially being the case in dense stands of mature grass, as the chemical is not reaching alot of the living plant tissue I would guess. Also they mention spraying in September, which by that time the NWSG is definitely mature and may already be semi-dormant. I am pretty sure that unlike trees which typically send plant reserves into the roots at the end of the growing season, grasses do so on a continuous basis when they are actively growing. Otherwise repeated mowing or haying would kill off grasses. We are more or less spot spraying, actively growing taller grasses in our Christmas tree rows.