But few Protestants know about that. And by what standard did he consider them not to be "inspired"? Is not anything written under the guidance of God's Spirit inspired?
Precisely. What I meant was that ideas about what the canon of Scripture is is not Scripturally-evident, it's something constructed out of tradition (look at how fast someone now would be rebuked and held to be in a cult if they produce a canon with different books or having subtracted and added ones to theirs). Whether you point at one of the ecumenical councils that decided this in the Early Church period, or you go with what Luther considered to be canon, you are still making a decision on inspiration yourself on which to believe.
IIRC, Luther's reasoning had something to do with what Judaism accepted as authoritative, but that gets into contentious waters because the Catholic position is that Judaism rejected those books post-Christ (actually around the time of the Siege of Jerusalem and Revelation being written), in order to mask Christian fulfillment of Messianic prophecies in those books. I'm not really sure.
But if it was quoted as scripture in Jude, which is considered to be scripture, then doesn't that call into question the book of Jude? But for those who consider scripture to be inerrant, that is unthinkable.
Well, they'd already reject the idea that Jude was in question, despite it being in the Antilegomena, which are themselves of contested origin. Adherence to pre-conceived notions rejects any sort of historical scholarship in those sorts of groups.
I'd just look at it as making a cultural backreference, just as if we were to draw an analogy from Lord of the Rings or something. The Old Testament has instances of it too, referring to legends or folk stories that the original audiences would have been privy to, but that we aren't because they didn't survive in either oral or written traditions. Like that blurb in Genesis about the Nephilim and the "men of great renown".
Even with those who subscribe to Inerrancy, they still have to have some measure of why they consider Luther's canon to be inerrant, and the deuterocanon or other books to not be. Then you start treading into a little historical revisionism, because I bet they'd say something to the effect that the modern Protestant canon was always the Scriptural standard, and there were no conflicts over what literature in the Early Church was inspired. But there's just too much apologetical evidence from that period against or for different books for that to be true.
The point I'm trying to make here is that a lot of people consider the Bible itself to be sacrosanct. I don't see it that way. I have faith in God to reveal Truth to us; I do not have faith in man to accurately transcribe it. I know there's a word for this, but I can't remember it: the idea that we should look at the gospels (the words and works of Christ Himself, testified to by his disciples who carried their testimony unto death) as a filter through which the rest of the New Testament should be viewed.
But that requires a distinction to be made between the Word of God and, for example, the words of Paul, which is something that most Christians are unwilling to do.
I'm not quite sure of the exact position you describe with Gospels primary and Pauline documents secondary, but the idea that things outside Scripture are the lens through which to interpret Scripture typically means the person adheres to Prima scriptura. You won't find Prima scriptura-ists among Evangelicals and Fundies, though, and for that matter, won't necessarily find them inside Mainline Protestantism (aside from Anglicans and Methodists, if the three-legged stool** and Wesleyan Quadrilateral are considered as endorsements of the Prima scriptura position; personally, I do consider them as such endorsements, but there's likely disagreement on that from the low-church portions of both schools, and mixed perception in the broad-church areas).
**Scripture, Tradition, Reason for Anglicans; Scripture, Tradition, Reason, Experience for the Quad