When I wrote the quoted statement, I was not referring to your original comment, but rather to your response above.
Again, such analysis would not be valid, because it would be based on isolated password exemplars (not on analysis of the generation process).
I have not proposed this.
As I tried to explain above, my statement about horse correct battery staple onion having 13 bits of entropy was based on an analysis of the process by which you came up with that phrase — it was not strictly based on examination of the passphrase components.
Now, to be fair, you did not disclose your method until after had done my analysis, so I confess that I did guess the process that was used, based on context clues (the fact that you had referenced the XKCD comic, and the vanishingly low probability that anybody would be able to randomly generate the exact phrase “horse correct battery staple”). So I would agree that my method for deducing the generation process was not 100% rigorous, and that this would be a valid basis on which to question my 0-bit valuation.
However, the 0-bit estimate for “horse correct battery staple” was subsequently validated, after you disclosed the process and confirmed that it was not a random choice:
In an alternative universe, if you had been collaborating with Randall on the XKCD strip about passphrases, and he had proposed: “Let me come up with a four-word passphrase, and you come up with an extra random word to add to the end” — and if the published comic strip used the example horse correct battery staple onion as a result of this collaboration — in that case, I would agree that the entropy of the 5-word phrase generation process would be around 55 bits* (or 57 bits, if you used diceware for your part).
*One can also legitimately criticize Randall’s 44-bit estimate, but that may be off-topic even for this off-topic side-discussion…