One of the first things my broody hens do when they bring the chicks off the nest is go to a bare patch of dirt. The chicks immediately get grit plus they are picking up bits of nutrition, some of which might need to be ground up. I personally do not see where getting git into their system at a very early age is a problem, just don't overdo it.
There may be some rational for delaying giving grit to certain brooder-raised chicks for two weeks, probably depending on how they are managed. I'd love to see where someone said why it's best to wait. Since we raise them different ways the why might be important.
I raise my brooder-raised chicks on wire and generally only feed them chick starter. I could probably wait but I want mine started on flock immunities immediately. If I raised mine on wood shavings or something they could eat, I'd want them to have grit in their gizzard so they can grind that up.