Its nothing to do with "if they're good enough they'll play".
Of course they will, any manager would play a young player if they're clearly very good and able to slot into the first team squad pretty much straight away, there's very little risk in that.
Its the other ones, the good young players who have potential but need to be given that chance in the team to succeed or fail.
They're the ones where the risk comes that a more cautious manager won't play.
A "safe" experienced pro who'll you know you'll get a 6/7 out of 10 performance from, or a young inexperienced player who might give you a 4/10 yet might give you an 8/10 and become a more useful player to the squad than the safe option.
Moyes would clearly always go for the former rather than the latter. Its debateable as top which is the best way of doing things but he clearly doesn't have a record that backs up any claim that he gives youth a chance.