I don't see the comment arguing for that at all, and I don't think the analogy to crop monocultures being more vulnerable to pests really holds.
There are good reasons we deride "security through obscurity" as valid, and just because "structural diversity" makes automated scanning harder doesn't mean it can't be done. See Shodan.
The idea as I (who is not GP) see it is not that diversity makes scanning harder, it’s that it makes the blast radius smaller. Notably, though, that means we have to be talking about diversity of implementations, not just deployments—numerous deployments of just a few pieces of software can be problematic in their own ways, and of course there have been bugs with huge consequences in Apache, MSRPC, or—dare I say it—sendmail since the very earliest days.
There are good reasons we deride "security through obscurity" as valid, and just because "structural diversity" makes automated scanning harder doesn't mean it can't be done. See Shodan.