That makes no sense. I've been told personally many times, and have heard it from the mouths of well-known sede clerics on videos, that those who go to 'una cum' masses commit a mortal sin. If this is an erroneous stance, (and it is, because it is not from the Church, but is invented), then such a error is also mortally sinful, since its accusations, it's scandal and its topic are of the same moral gravity.
What if someone called something a mortal sin for a certain reason, but it was wrong....but nevertheless was a mortal sin for another reason?
This is the case with the "una cum" threat. It is wrong in considering it a mortal sin for an intrinsic reason, but is still correct for considering it a mortal sin for an extrinsic reason. Here is the quote that proves it:
"Since heresy, and any kind of infidelity, is a mortal sin, they also sin mortally who expose themselves to its danger, whether by their association, or by listening to preaching, or by their reading." - St. Alphonsus Liguori