It's a riff on the "It's never/always DNS" meme. Pointing out the common inconsistency between reality and expectations of when the issue you are facing has is due to DNS.
So it's a double cancellation of it that bring it back to the original phrase? To show that they didn't take it seriously because it isn't actually true, but then they found it was true in this case, or at least it felt true if all other cases were ignored? Like it was sorta true but not completely since another problem might have had a non DNS issue at its source?
I feel like I see what they're saying but I'm still confused at what's getting communicated. Just "sometimes DNS can actually be a source of problems?"
It's probably really only "funny" if you're familiar with the meme. In that way, it's like many inside jokes. You can't really logic it out. It's like, when someone explains a joke to you, you can now understand why it's funny, but you can't put yourself back in that place where the joke would hit you with the intended impact. Don't worry about it.
https://www.cyberciti.biz/humour/a-haiku-about-dns/