The difference is that Bp Fellay was alive in 2009, and Abp Lefebvre was not. Excommunications are medicinal penalties which only apply to the living.
So you believe the excommunications were valid? Yes or no?
Note that all four bishops had their excommunications lifted, not just +Fellay.
It could not have been done otherwise. There was no difference in the cases of the four bishops, and there was no "repentance" - at least not officially.
We really have no idea what Bishop Fellay has promised.