She was required for Jesus to become the fullness of man, of course, so her obedience was necessary for our salvation. But Co-Redemtrix sounds to their ears like she's up in Heaven, ironing His shirts, packing His lunchbox, and determining whom to invite to the Feast. I consider the term unclear and misleading. You have to be steeped in the writings of the saints of old to even really truly comprehend what it even means, and the resulting concept you glean from that study is underwhelming.
Perhaps SSPX should see Mary as an example of Obedience to the King filling a person with Grace, and go and do likewise.