Let's try this again.
Please don't sinpost this time around.
What defense do the Catholics use today to justify treating the council at Constantinople in 869-870 as ecumenical over the council in 879-880?
Modern scholars agree that the council of 879-880 was much larger, had representation from all 5 patriarchates, ended the schism that was opened ten years earlier, anathematized the council of 10 years earlier, and was agreed to by the Pope. In fact, it was not "replaced" by the council of 869-870 until the Gregorian period, when the rediscovery of canon 22 of the council of 869-870 led to the council of 879-880 being forgotten.
So, the council of 879-880 had all the potential characteristics of an ecumenical council, and it anathematized the council that the Catholics call the 8th ecumenical council. Are there any modern arguments for the ecumenicity of the council of 869-870 over that of 879-880, or is the issue just swept under the rug?
If the council of 879-880 is the true ecumenical one and the council of 869-870 is only called ecumenical by the Latin Church due to a mistake of history, then that changes the dynamics of the schism completely.