I read the whole bit, made sense from a biologist's (an old one like me anyhow) perspective when colored by the needs of the reservoir manager/city and budgetary concerns.
Grass carp is a management tool just like a powerful herbicide having a long lasting residual effect. Especially nowadays with sterile (triploid) grass carp as the only one being utilized. But long lasting isn't forever, and like a powerful herbicide they can "drift" and have potentially undesirable side effects in other places. But what effects are "desirable or not so" is more to do with the observer than anything else (fisherman vs. lakeside homeowner).
Hydrilla is a non-indigenous weed that sometimes provides suitable habitat for native and non-native species, some of which are desirable to sportsmen, some of which are not. Just like grass carp, tilapia love the stuff as food and cover both. This may or may not be good for the local bass, which usually has little to do with whether or not it is good for the local bass fishermen.
In my life as a fisheries biologist I've found that for the most part, bass fishermen just want to catch a lot of really big bass, really easily, in a short period of time when its convenient to them and without having to go too far or spend a lot of money. Pretty much anything at all that they perceive to be an impediment to this, whether it is or not, they just don't like. Stuff like work, broke down truck, wife wanting toilet fixed, tilapia and grass carp. OTOH they like things such as, happy wife with lots of brownie points in the bank, cheap gas, buddy that likes to fish and spend money, new boat that works good, state hatchery bass stockings and hydrilla.
Kinda like a herd of goats, grass carp will eat the stuff they like the most first (hydrilla, elodea, najas) and when its gone they eat stuff they like less (cattails, carpetgrass along the edges, duckweed) then stuff they don't care for at all, but eat instead of starve to death (milfoil, coontail, hyacinth) some stuff they won't eat at all (chara, filamentous algae, planktonic algae).
So, now that the carp done ate the hydrilla and the water's greened up, the shad and sunfish'll do really good, the bass'll eat a lot, and when the carp dies off and the weeds come back and the water gets clear again, a buncha whoppers'll be waitin' to be caught.