Onana was an important geometric parameter. His position determined the angle for the strike. By moving one way or the other, he opened or closed that angle. By moving at all, he changed the calculus for the keeper.
Do you think Barkley had the time to even consider that Onana might be a useful 'shield'?!! It all happened very quickly. And, there was also a defender next to Onana acting as a shield.
The key point is that the ball did not brush him ...
No, but that's not hugely relevant, I don't think.
And yes, apparently, as the laws of the game would have it, the only important thing is whether Onana got a nick.
But I would have been seriously gutted to see one like that given against us. Wouldn't you?