If he did, he may have thought it was a worm, and considering he "swam up", couldn't use his whiskers to deny his feeling. He probably spat it out.Yes, corys do eat poop. I have a panda corydora. I am new to this species and I wouldn't have beleived it if I hadn't seen it with my own eyes. My cory, "Hoover", swam up and ate some comming right out of another fishes bottom. So, yes, would have to be the answer.
Would you like to know what a cory's whiskers are used for? Identifying food, IE some rotting plant matter, dead fish, underwater bugs, etc from objects. The whiskers taste. That's why cories rest in front of food before eating it. They're checking if it's food. They have those whiskers for telling the good stuff from mud, branches, pebbles, and yes, POOP....and their niche includes browsing on whatever their whiskers touch and identify as good nutrition. With almost 100 fish in my aquarium I rarely see fecal material and my 9 Corydoras are constantly browsing on the bottom...
...Although your post did sound convincing ...
If you plan on keeping it, get at least 3 more. They should start schooling nicely. Buy some algae wafers and/or shrimp pellets. The guy at the LFS is either A) an idiot that probably doesn't care about fish, just the money, or B) is misinformed and, like all but 1 of the people I've really met in my life at a pet store, knows nothing but thinks he does. He is obviously not very smart, because I'll be surprised if you can pull up a site that cories don't like company.
-DP