The setting is Barnaby Sheen's house in Tulsa, not a Greek taberna, and Napoleon is not mentioned, but I believe you're thinking of R. A. Lafferty's 1974 short story "And Read the Flesh Between the Lines". It was first published in the anthology Universe 4 (Terry Carr, ed.), which can be borrowed (for free but registration required) from the Internet Archive: 1974 edition, 1975 edition.
Review from the site Science Fiction and Other Suspect Ruminations:
"And Read the Flesh Between the Lines" (1974), R. A. Lafferty, 4.5/5 (Very Good): As expected, Lafferty weaves the most challenging story in the collection! First, imagine history as one would a memory–compressed, selective, porous, constantly rewiring itself. Now imagine a physical manifestation of memory—a throbbing room filled with ephemera of youth and the items of nostalgia and the language of comic books… This proximity of images collides with what could be an alternate-history, as a man ruminates out loud with his Australopithecus servant serving drinks. Memory as a passive myth-generating process? I want to reread this one and re-uncover its threads!
The people having the conversation are Lafferty's Men Who Knew Everything and the author himself:
The four of us, Dr. George Drakos, Harry O'Donovan, Cris Benedetti, who were three smart ones, and me, who wasn't, went down and examined the old room.
Barnaby Sheen expounds his theory of history:
"There is the leaky past, but it cannot leak out fast enough for safety," Barnaby had taken up his tale again. He always came as directly as possible to a point, but the point was often a tricky one. "The staggering corpus of past events, and of non-central or non-consensus events, is diminished swiftly. More and more things that once happened are now made not to have happened. This is absolute necessity, I suppose, even though the flesh between the lines (it is, I guess, the supposedly expunged flesh) should scream from the agony of the compression.
"Velikovsky was derided for writing that six hundred years must be subtracted from Egyptian history and from all ancient history. He shouldn't have been derided, but he did have it backwards. Indeed, six times six hundred years must be added to history again and again to approach the truth of the matter. It'd be dangerous to do it, though. It's crammed as tight as it will go now, and there's tremors all along the fault lines. As a matter of fact, several decades have been left out of quite recent United States history. They should be put back in (for they're interesting, and we ourselves lived through parts of them) if it were safe to do so."
[. . . .]
"But when you try to compress a hundred thousand years of history into six thousand years, something has to give. When you try to compress a million years, it becomes dangerous. An involuted number series, particularly when applied to the spate of years, becomes a tightly coiled spring of primordial spring-steel. When it recoils, look out! There comes the revenge of things left out.
"Were there eight kings by the name of Henry in England, or were there eighty? Never mind; someday it will be recorded that there was only one, and the attributes of all of them will be combined in his compressed and consensus story.
[. . . .]
"When were the several decades left out of United States history, Barnaby?" Cris Benedetti asked him.
"Early, and recent, and present, for I rather suspect that our own contingent present will not be firmly inscribed in the records."
"You mean that we may not be recorded as real?" Drakos asked.
"Possibly not," Barnaby said."I'll give one example: there is the case of a father, son and grandson from one family, John Adams, John Braintree Adams, and John Quincy Adams, all being President of the United States. I notice, though, that only two of them are now believed in, or are now written in. The best of the three (wouldn't you believe it? it's always the best) has been left out. But the foreshortening was continuous, and part of it, I believe, took place during our own boyhoods. There was much more happening then—three times more—than we are allowed to remember. Sometimes it seems that it was a million years and not just a couple of decades left out here."