your chef friend is right and shes not talking about diced tomatoes she probably means the steamed whole ones
they wont send the ripe ones to market because they will take too long in transit so they send the unripe ones because they will ripen during that time of transit and storage and also they get frozen and unfrozen
the canned ones get flash steamed or whatever, have some salt added to preserve them and are ripe right there in the can. they are generally much more flavorful for cooking with
if you know someone growing heirlooms those are the best option for fresh if you can get them off the vine they will blow the socks off any store tomato