It's actually quoted from a discussion by Aquinas, which is more complete and does not exclude the idea of a non-Catholic who believes he is acting without fault being saved. This was common knowledge at the time, not a novelty. (Trent affirmed it, Pius IX affirmed it, Vatican II affirmed it ... it's old stuff.)
Re: The definition of anathema, that would normally be how I read it too, but the RCC has long claimed that what its popes bind on earth will be bound in heaven, in accordance with its (erronious, in my view) interpretation of Matt. 16. Therefore, for a formal conclave of the Roman church to anathematize someone is as good as them saying, "We're condemning you to hell."
Re: Unam Sanctam, the contextual argument is made rather irrelevant and frivolous by the absolutist and universal language the Pope chose to employ. He does not say that "all Catholics must be subject to the Pope" (which would be both fair and true), but "all human beings must be subject to the Pope" in order to be saved.
In any case, whether the anathema of the Protestants is the RCC attempting to sentence them to hell or whether it's the RCC "recognizing" that they're going to hell doesn't really matter to me. Honestly held exclusivist theology has never been my sore point. What does matter is that until that "sentence" and/or "recognition" or whatever you wish to term it is revoked, it makes any ecumenism between Protestants/Evangelicals and Catholics a total farce. And it is that farce that concerns me, not simply the beliefs and teachings of the Roman magisterium.
Of course, the Catch-22 is that the RCC cannot revoke so many of its own councils and bulls without completely undermining its own authority. It is much, much easier to simply try to bury such language and rewrite history to edit out the polemic of both sides in the hopes that the vast majority of Christendom, Protestant and Catholic alike, simply remains ignorant of history. Those of us who know our history don't much care for that. Furthermore, no dialogue based on diplmatic spin, lies, and omissions could be pleasing to our mutual Lord and Savior.