The Archbishop worked with other sedes anyways, so it's just a moot point.
I disagree.
Granted, what you say is probably true and has long been understood by most sedes (and a handful of Ecclesia Dei trads like myself).
However, in light of the R&R mythology of Mgr Lefebvre as a consistent and principled anti-sede, his public cooperation with a sedevacantist Bishop de Castro Mayer -- especially in the matter of the liturgical consecration of bishops -- raises several questions.
As you rightfully say, it's mythology. They follow a narrative, and hold to it like gospel truth. Look at this issue of Bp. Castro de Mayer, it has been a long accepted fact that he was a sedevacantist, but it doesn't fit the narrative.
The same with Archbishop Lefebvre, it is commonly known that he saw sedevacantism as a real possibility, and many believed that he was most likely privately a sedevacantist by the mid-80's. But again, it does not fit the narrative.
The narrative must be defended at all costs.