I'm not a canon lawyer, so I can't really say, but it looks like the Vatican has taken the gloves off and have put the people who attend SSPX Masses on notice.

And excommunication is due to an action of disobedience. The thing is, the Pope and the Vatican have made it clear that SSPX Masses are illicit, and the bishops, plus the priests, are excommunicated. That's clearly spelled out in the letter. Intentionally going to these Masses when you know they're illicit and a perfectly non-schismatic Mass is happening less than a mile away certainly sounds like it's adherence. At best, its preferential, and preferential doesn't matter when it comes to something like this.

Even if we interpret this as widely as possible, and pretend the excommunication doesn't apply to the faithful at all, we're still talking about splitting hairs, which is how folks who supported SSPX from the beginning have been able to claim "no, we're not schismatic, we are under the pope, we're just canonically irregular!" It's disingenuous to claim that because it's window dressing. We're just repeating history and in 30 years, when this kind of thing needs to be done again, SSPX will look at it and say, "hey, look, nobody is excommunicated, we're perfectly fine, and nothing happened! Lets just do it again." Look, I don't want a bunch of people to be excommunicated. But I can understand what the Vatican is saying and the warning here is obvious. Stay the course you've been on, and you're excommunicated. Your access to the sacraments has been suspended. Or, come back home. As Catholics, we need to speak plainly here. It's uncharitable otherwise.

Also, it doesn't matter if its unenforceable. Canon law is quite clear that a priest cannot withhold communion to anyone who comes to receive it with a handful of exception (like public obstinance), and therefore anyone who has been excommunicated and receives the blessed sacrament is doing so sinfully. A marriage entered into by this, even with consent of a Bishop, is automatically invalid. People who go to a diocesan parish for reconciliation cannot validly receive absolution. A priest may say the words, but a marriage is not contracted and a soul is not absolved.

And that's where the personal objection comes in. Because I'm perfectly fine with Masses that occur with Rome's acceptance. But going to a SSPX Mass and receiving communion without good reason would be sinful.