The article talks about the possibilities of malicious cloning of these tokens by third parties, but fails to identify the much more common use case, and one that makes this scheme useless for age verification.
It's one thing to be concerned about someone stealing my credential, but another to prevent the transfer of these credentials, especially if they are limited use credentials.
The entire point of age verification systems is to prevent minors from accessing certain resources. I think we all know that this is basically impossible; but what these various governments and social media companies want to do is to make it high friction to do so.
The highest friction version of this is that the credential ties to a real world identity somehow; maybe locked behind legal barriers, etc., but if a minor is caught using someone's credential, then the person whose credential they are using can be investigated, and, if necessary, charged with a crime roughly equivalent to providing alcohol to a minor. Without the possibility of real world enforcement, none of these identity solutions can possibly work.
Keep dreaming of a technological solution -- there is none that does not lead to the world that FIRE is warning about, except to accept that we can only make a solution "good enough" and leave it at that, without expanding into full on identity verification. The solution here is likely to just try to provide better abilities for parents to monitor and limit their children's use of the internet. Let individual parents decide on the level of harm that they are willing to accept, and accept that there will be ways to work around this even if parents are vigilant, but just try to reduce it on the margins.