If you have seen no other Discus at the store,,could have been that someone brought the sick,malnourished fish in,, and store folk's just could not euthanize the poor creature though it is what I would do.
Could be the poor fish came in with a bunch of other young Discus that have been sold , and somehow missed being culled and again,,the folks at store just tryin to keep from killin somethin at Christmas
Worst case,,,they had one specimen shipped to them to gauge public interest, and you could take comfort that they won't be offering more after witnessing the result's of their misguided efforts or lack thereof.
Puppies I have raised, were not offered for sale unrtil they were around eight weeks old and Parvo shots or at least first shot's had already been administered,
Sadly,,lot's of Puppy mills here in Missouri.