Of course this won't be 100% effective, maybe 80-90% effective. That's all they need and expect from this system.
HN is constantly obsessed with is it perfectly effective?
No law, none, is perfectly effective. Speed limits certainly aren’t self enforcing, but remove your neighborhood’s speed limits first if you truly believe laws must be demonstrated perfect.
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.