Yeah those chicks in the bargain special you mentioned are specifically NOT cornish x rocks. They are generally unwanted cockerels that the hatchery wants to get rid of due to their limited market value.
If you just want some birds for the fun of it, and don't mind waiting a few months for some marginal meat yield then go for it.
I have friends that buy these assortments and let them free range all summer and then butcher them for soup birds, etc.