The WHO says that we can't assume someone is 100% guaranteed to be immune when they have antibodies. That's true, and it's an important flaw in the idea of immunity passports that they're tackling - you couldn't send people with immunity passports into quarantined nursing homes or allow them to attend potential superspreading events.

Herd immunity doesn't require perfect immunity or a 100% guarantee of it. No reasonable expert doubts that herd immunity is possible, although some argue it's too costly.