Unfortunately, the FSSP signed an agreement with the devil, oh sorry, I meant the Vatican, saying they would con celebrate one Novus Ordo Mass per year with the conciliar Church, the Chrism Mass. You make a deal with the devil and you always lose, as he is the father of lies.
You are wrong. The FSSP never signed such a thing. The FSSP celebrates the latin mass exclusively, as already said in the comments. Some priests did con celebrate in the past, and they are no longer (since more than 10 years ago) part of this community.