The author did not claim that Delta's origin was a lab leak. Just that there was one isolated case of it. And so it stands to reason, other lab leaks may also be occurring.
We should note you're talking about covered up lab leaks where all of the participants are conspiring to keep it secret because of convoluted motivations. It doesn't "stand to reason" that this happens at all, let alone is the normal case that we should all assume.