I agree that in D&D orcs are not being used as an allegory or replacement for one kind of human being. At the same time, the way orcs (and drow, and elves, and halflings...) are described uses the practice of ascribing the values of a culture to one singular "race." And whether this race or culture is real or imaginary, the act of ascribing the values of a culture to a single race is, in itself, a harmful practice.
The big difference to me is in the following two models:
A) orcs are barbaric raiders
B) there are barbaric raiders, and some of them are orcs
In example A, the "race" of orcs is being defined by the values of a culture (barbaric raiders). In B, the values of a culture are separated from race.
Obviously no orcs in real life are being harmed by being depicted as barbaric raiders. But I would argue that the very act of ascribing the values of a culture to a single race is harmful, and it's harmful to those who act it out playing the game because it's an unhealthy practice.