I think symfony/symfony#59106 (comment) invalidates this

Nah symfony/symfony#59106 (comment)

Another valid reason to use __serialize is that a real-world project might have additional relations in their User entity (for other reasons that the authentication) and you would not want to have all of them included in the serialized payload (which might even trigger initialization of proxies depending on whether serialization triggers it or no).
And if serialization does not automatically triggers the initialization of proxies, you would need this __serialize performing property access that would ensure that Doctrine initializes the proxy.