If I had hoped Mr McIntyre would not identify the source of my very funny metaphor and would think me responsible for its brilliantly humorous simile, I would not be a brilliantly humorous writer, I would be a dumb and contemptible plagiarist. And if I had thought he would spot the quotation but I was wrong and he did not, I would be in an awkward spot for two reasons: (i) I would have gratuitously insulted someone I didn't even know, and (ii) I would have used someone else's clever humor without admitting it or citing the source, and would thus have put myself in danger of being fingered later as a plagiarist.Comedy's complicated. I think the concern is not so much plagiarism -- though Pullum is setting up an interesting plagiarism anecdote -- but the problem of seeming too weird or crazy. If you refer to things that you know everyone knows, it's going to be trite and tiresome. If you refer to things almost no one knows, you're going to be irritating for another reason. Allusions need to fall in some middle ground where the other person is going to get it and feel good about himself for being savvy enough to get it. That said, when you're writing on the internet, the middle ground gets much larger. When someone uses an odd phrase that you don't recognize -- like, for me, Pullam's "a hundred clowns with bees in their underpants" -- you Google it and then you can give the impression of someone who is savvy enough to get it.
But I had judged him right: I took him to be well acquainted with such familiar features of our culture as the Dilbert strip, and I intended him to see that I was quoting, and he did, and I intended him to see that I intended him to see that I was quoting, and he did, and I intended him to see that I intended him to see that I intended him to see that I was quoting, and he did, and... Perhaps it would be simpler if I just cut this (non-vicious) infinite regress short and say that I intended there to be not just recognition of the quote but also mutual recognition of our mutual knowledge state.
Googling the odd phrase is also the way to detect plagiarism, but now when you find the phrase by Googling, you've got to ask whether they used it because they thought you'd Google it so you could consider yourself savvy for getting an allusion or whether they just didn't realize you'd Google it. Pullum's plagiarism story tells of all the excuses he heard when 10 students plagiarized the same quote they found on the internet (including a ridiculous one I've seen a student get away with: "I didn't realize it was wrong because in the country I come from copying is quite normal"). But they didn't try the one his post suggests: "I assumed that you'd Google it, and then appreciate my allusion."