They're in a slightly different position from the SSPX because the folks involved have been a separate denomination for quite a few centuries.
Having started out as a Piskie myself, I can tell you it's quite a come-down to acknowledge that all those sacraments you thought you had for decades didn't amount to a hill of beans (except of course baptism). THAT is a much more serious thing to accept (especially for a priest) than acknowledging the validity of what you were supposed to have sworn obedience to in the first place.
Correction: They have to be validly ordained since they never were as Anglicans.