In short, the answer is that nowhere did the Fathers of Florence - that is the context of
Cantate Domino - define what it means to be "outside the ecclesiastical unity" of the Catholic Church.
The article Steve linked to demonstrates that there is, in fact, a difference between "being inside the Catholic Church" and "being a visible member of the Catholic Church." The former is absolutely necessary for salvation, while the necessity of the latter for salvation is
not absolute.
As the article puts it:
That last assertion, then, is the key:
the maxim “outside the Church there is no salvation” does not imply, as it seems to at first sight, that “only inside the Church is there salvation”.
In support of this argument, Harrison takes another look at the quote from
Cantate Domino and basically asks, "What about catechumens?"
They have not yet been received into the Church; indeed, they've not yet received any Sacraments. They are
not yet "members of the Church," yet they're not "outside" the Church either.
Conspicuously,
Cantate Domino does
not list "catechumens" in its list of people who cannot benefit from grace. It lists, rather:
(1) Pagans
(2) Jews
(3) Heretics
(4) Schismatics
The fact that "catechumens" are omitted suggests that the document does not intend to exclude from them the possibility of salvation. Since they are not yet members of the Church, therefore, it follows
even by the standards of Cantate Domino itself that "visible membership in the Church" is
not necessarily a prerequisite for salvation.
Likewise with
Unam Sanctam: one can be "subject to the Roman Pontiff" in other ways than visible membership in the Church. Say someone, if he knew of the papacy,
would desire to submit to the pope. It seems Tradition says that can be sufficient.
Harrison makes this very point in this article: if in danger of death, it is lawful to baptize someone even if they only know about and profess (1) the Trinity and (2) the Incarnation. Such a baptized person on his deathbed would certainly be "subject to the Roman Pontiff" without knowing it.
In short, I now feel I know how
Cantate Domino fits within Sacred Tradition.
If anyone is still confused or uncertain, then for the sake of context and comprehensiveness I wholeheartedly urge you to read the entirety of
Harrison's article.
How does everyone else feel? Are you satisfied by Harrison's explanation?