Not a myth but historical reality. The only thing is that the presidency was never as active as it should be because for the most part they couldn’t speak Greek. So someone would generally fill in their role for them, either an eastern bishop or normally the emperor
The popes were actually not as ignorant of Greek during this period as you might think. Furthermore, there was a large Greek community in the city of Rome, particularly during the Iconoclasm period. It’s difficult to imagine that any representative from Rome during this period would have been entirely ignorant of Greek.
II Nicaea was not presided over by a papal legate, but Patriarch Tarasius and Empress Irene. It’s abundantly clear in the acta. The council is opened with a speech by Tarasius. See page 1 of the translated acta here:
books.google.com/books?id=5sCqMrxtjBAC&printsec=frontcover&dq=%22seventh+general+council,+the+second+of+Nicaea,+held+A.D.+787%22&lr=&hl=en#v=onepage&q&f=false
Furthermore, the final session was directly presided over by Empress Irene. The Franks knew this too, and attacked the council precisely because of such (pardon my rusty Latin):
“We ask therefore the Apostle! The preeminant preacher says, the vessel of the chosen says, does he permit women to teach or to rule over men? He says, in fact, to Timothy: ‘Women shall learn in silence with all submissiveness. However, I do not permit women to teach, nor to rule over men, but to be in silence (1 Tim. 2:11).’”
“Interrogemus ergo Apostolum! Dicat egregius praedicator, dicat vas electionis, uturm feminas docere permittat an in viros dominari? Ait enim at Timotheum: Mulier in silentio discat cum omni subiectione. Docere autem mulieri non permitto, neque dominari in viram sed esse in silentio.” Theodulf of Orleans, Opus Caroli Regis contra synodum, MGH Concilia II Supplementum, I pg. 387-388.
Again, the Franks questioned the authority of the council precisely because a women presided over it. Papal representatives attended and played a crucial part, but they did not preside over the council.
Correct me if I am wrong, but I do not think subscribing to Catholic papal primacy claims necessarily requires that a papal representative preside over a council. It only requires that a papal representative attend and approve.