Technically, none of these sacraments is valid, but do you think God will just ignore it, not give the people grace, and let them go to Hell because of a technicality?
Privately, I of course have good hope He would not ignore it.
But we can't PRESUME extraordinary interventions or exceptions, certainly not publicly. That's the whole problem with the new ecumenism that the SSPX (rightly) hate. For example, most protestants these days are only material heretics. They may not be culpably ignorant. So God might use, for example, their baptisms as a means of salvation. But the key word is might. It is a private hope, not the certainty of faith that public revelation obliges us with. It is presumptuous to say that God will certainly do so, or to act in a way that seems to just assume that He will. That leads to indifferentism and lessens the urgency of evangelization and baptism. That's the problem with the new ecumenism. But so too, in the public forum, positively counting on (read: presuming) the idea that God is going to make such extraordinary interventions...is as audacious as not repreforming those hypothetically invalid sacraments once the mistake is found out. Counting on an exception to the rule, presuming extraordinary means...is exactly what we cannot do exactly because that is what makes them extraordinary. They are a matter of hope after-the-fact, and if they occur (and I have good hope they do) a mercy of God...but certainly not something to be presumed as if certain like a rule. If you start to think of exceptions to the rule as consistent parts of the system of rules...then you are no longer viewing them as exceptions, and that is what presumption is. If you start to assume God's mercy and so sin just because you know He will give it...that's an abuse of the system and is the very essence of presumption. You can't presume mercy, and you can't presume extraordinary means, even though God does give both.
God's grace is not restricted by the ordinary means when good will and accidental circumstances come together. No one is damned except by their own bad will.
But presumption is one of the sins against the Holy Spirit.
Also, if real schismatics and heretics like the Orthodox have valid sacraments, what makes anyone think fake schismatics would not have it?
The conclusion seems to be that it is because they had pre-existing jurisdictions that broke off corporately. But the SSPX only ever derived their jurisdiction from the Pope, as an apostolic society.
If they had been a personal prelature before the schism...I could see the argument. But they werent. They had no jurisdiction of their own beforehand.