Most of my readers are certainly familiar with the concept of supplied jurisdiction, so I will waste time on this. I would like, however, to make some considerations for the “legalist” crowd.
So, you think the SSPX has no supplied jurisdiction, and any confession or marriage celebrated by them is not valid. Why would you, then, go to confession by them after the 8 December and for around one year afterwards? Why would you go to confession to people you consider abusing their priestly role by doing things they are explicitly not allowed to do, and even deceiving the sheep about the validity of their confession or their marriage?
Apart from very rare, rather extreme examples, in which I believe the SSPX priest has jurisdiction even for the Vatican (say: man about to die, no Novus Ordo priest around for the Last Rites) the answer can only be one: because they are so beautifully, authentically Catholic.
Which, truly, answers the question about the supplied jurisdiction once and for all. These are perfectly Catholic priests doing nothing else than perfect (ahem, almost perfect if you ask me) Catholic work. There can be no doubt on their being 100% orthodox. There can be no doubt on their obedience to ecclesiastical authority whenever this does not contravene to a higher loyalty, the one to God. There can be no doubt the metre with which the SSPX measures this higher loyalty is nothing else that Catholic tradition in its purest form, as opposed to the orgy of Neomodernism (or worse) en vogue in Rome.
How can you, therefore, say that these disobedient people are your example of Catholicism, which is, in the end, obedience to the Lord? Are they being obedient to the Lord? Then they most certainly have supplied jurisdiction. Are they not obedient to the Lord? Then you should not approach them during the Year of False Mercy.
Ecclesia Supplet. When a Catholic is not allowed by his bishop or his Pope to do the Catholic thing, he does it nevertheless, with many greetings to the Pope or bishop. Athanasius did it, Eusebius did it, and Marcel also did it. I am pretty sure many others simple priests found themselves in similar situations and did the right thing, particularly during the Arian troubles, because I can't imagine that there weren't many priests ready to say exactly this: Ecclesia supplet. If you are a truly Catholic priests, and your bishop tells you that you must recite a different Creed at Mass – because some of the pewsitters might be offended by the old one – you know exactly what directions you will give to him, and that's that.
The SSPX have supplied jurisdiction, or they haven't. If they have, you certainly don't need Francis to tell you what they can or cannot do. If they cannot they are at least disobedient and probably gravely sinful people, and then you should avoid them anyway.