But under that argument, you would have to prove your age on a regular basis, the plan right now appears to be that each account would only need to do so once.
If transfer of accounts is a policy violation, then Discord has legal cover to confidently assert that, once ID is verified, the ID'd person is the owner and controller of the account thereafter.
Account selling, stealing, and sharing will certainly still happen, but that's grounds for banning, and not Discord's legal liability anymore.