@Azarilh I do not have the time to review and speak about this specific product sadly. But in general, even if the token handed to the application requesting it is fully anonymized, the application collecting the initial data is still a potential attack vector and point of failure.
If it's proprietary, then it entirely relies on blind trust. If it's open source, then it must be fully audited regularly and built and reviewed with independent experts. But even if it was perfectly secure and private, the piece of ID showing the age must be uploaded somehow. Is the whole system secure? Where is this data stored? Does it get fully purged after or is the "deleted" information only flagged as deleted but kept in a database somewhere?
If all identifiable information is fully deleted, then what shows this token is reliably only used by an adult and not shared with a child? Where is this token stored? Can it be sold to others online? People have already done that with the supposedly secure and supposedly private World App. If identifiable information is kept to prevent this, then all the other problems mentioned above remain.
And regardless of all of this, having to upload an official ID, even in the imaginary scenario where we would magically have a perfectly privacy-preserving technology, gatekeeps the use of devices and access to information and communication from many people who, for various reasons, cannot have this official ID. It closes down the internet. We should never agree to that, let alone contribute to facilitating it. More information here: https://www.eff.org/issues/age-verification