How deeply embedded is this false notion?
And he said this AFTER the excommunication had been lifted. What is going on with these people?
VERY, VERY deeply embedded.
I don't know - more embedded than it ought to be, evidently.
Still, wacko notions on the Holocaust, flat earth, or space aliens on the part of various individuals or groups over the ages don't necessarily invalidate correct beliefs in other areas. In the case of the SSPX, preservation of Sacred Tradition and rejection of modernism/dilution of Catholicism by Protestantism are correct and admirable postures, IMO.
Regarding the extant of Holocaust denial, although I have never attended an SSPX Mass, I have communicated at length with several individuals who have. These particular persons harbor no antisemitism whatsoever and would find such a position to be abhorrent. It would be entirely alien to them as it would to me. I personally think some SSPX homilies should address this situation, just as other human faults can be addressed as moral themes.
“How deeply embedded is this false notion?”
Unfortunately it is as likely to be deeply embedded within the SSPX as it is among other Christians and people in general. The news media is making an effort to sniff out the wackos among the SSPXers this week. Next week it could just as easily be someone else. Anti-Semitism has, I believe, declined greatly among Christians during my lifetime; but it still has a long way to go.
It was a finding of the Nicene Council 325 CE !
shalom b'SHEM Yah'shua HaMashiach Adonai