Or to the Orthodox who would use the Council of Constance as an argument against papal authority, for that matter...
Either [a] one of the claimants was the pope and he resigned for the good of the Church, or [b] there was no legitimate pope to begin with.
The pope could resign on his own initiative, seeing that it was practically impossible to adjudicate between the competing claims. This does not mean that the council itself was responsible for deposing the pope, or that it has a greater authority than that of the pope. The council would be taking credit for something that was not due.
