Since the bass you caught was keeper size and just slightly thin, it may be a good sign.
For ponds that have no shad population, bluegill are the main forage for bass. If there aren't enough bluegill available to sustain the bass population, then you typically won't catch a bass like you did. All of the bass will be stunted, in the 8" -11" size range with the exception of 1 or 2 really large bass that are the queens of the pond. If the bass in that pond had been stunted, you would have caught one every cast and probably never caught one the size you did.
Now, if the bluegill population was stunted, the limited number of bass in the pond would be fat & happy. They would have an enormous amount of small sized (stunted) bluegill to feed on. The bass you caught would be able to feed to his hearts delight and most likely would have had a pot belly instead of being slim. So again based on the fish you caught, most likely the bluegill population is not stunted.
I would wager that there is a healthy balance in that pond. The bass will range all sizes, but not too many will have pot bellies. The only bluegill you will catch will be the over sized ones, too big to fit in the mouth of all but the largest bass. The number of smaller bluegill is sparse, for they are constantly being eaten by bass like the one you caught.
A tip & a suggestion. The tip is that the Ned rig & other small baits will produce well in a pond like this full of northern largemouth. That matches what they are used to eating and doesn't look out of place. A big giant swimbait may catch the queen of the pond, but is a bit of a unicorn as for as matching forage. The suggestion is in regards to the health of the pond. If the homeowners want more bluegill & crappie around for the kids to catch, recommend that they do a couple of stockings of crayfish. The bass will often choose to eat the crayfish over the bluegill and if they establish a large enough population, the bass will back off decimating the bluegills so that a decent amount of mid sized bluegill end up surviving (to be caught be the kids).
And don't worry about the Koi. He can be the pond mascot, he won't affect the wild fish population.