It’s not likely to be a continuity error
I’m not quite sure what this means, or why it’s there. I can’t really explain it.
It doesn’t make much sense as a continuity error, because the prevalence of magic was established in that very book many times before that scene.
For example, Sandry knew that her room was hidden by magic:
“I’ll go crazy,” she said flatly. “When they come to rescue me, I’ll
be raving mad.” She refused to admit that, with this room locked from
the outside and hidden by magic, a rescue was hopeless.
She talks about being “no mage,” so she knows what a mage is:
“I’m no mage,” she argued, resting her head on one hand. “I’m just a
girl—a noble girl, worse yet. Like that maid said, ‘Good f’r naught
but to be waited on and to marry.’ Good-for-naught, that’s me—”
Tris was tested for magic:
“You were tested for magic?” he asked, his clipped voice abrupt. Why
did this stranger taunt her? Her family would have put up with her
oddities, if only she’d been proved to have magic, which might be
turned to the profit of House Chandler. “By the most expensive mage in
Ninver, if you must know. And he said I haven’t a speck of it.”
What’s more, Briar himself acknowledges the existence of wizards in his narration before that scene:
Briar frowned. “Master” was a word for professors, judges, and
wizards. The temples called women and men “dedicate.” Who was this
For Pierce to forget that magic was commonplace while writing this, and then to remember it some pages later, boggles the mind.
Another half-explanation presents itself, but it’s not very satisfying.
Briar is dismissive of magic
The best explanation that I can think of is that Briar was dismissive of magic, even though he believed in it. We see this attitude, too, before the scene in question:
Niko cut some fish for himself, put it in his mouth, and chewed it
carefully, without looking at his companion. When he’d swallowed, he
added, “It’s also one of the two great schools of magic north of the
Pebbled Sea. I studied at Lightsbridge, the university for mages, but
in some ways I find the mages at Winding Circle more … open-minded.”
“Oh, magic, who cares?” Briar dug into his food, refusing to talk more
until it was in his belly, where no one could take it from him. Plants
from all over the world? What must that be like?
Briar isn’t thinking “magic, how could that be?” but rather “magic, eh.” This shouldn’t come as much of a surprise. Mages are quite rare, and primarily retained by the rich and powerful, not homeless thieves. Magic likely hasn’t been all that useful to Briar (except when it gets in the way of him stealing things), so he doesn’t care about it much.
That said, this doesn’t constitute a full explanation. He might not have any regard for magic, but that doesn’t mean he’s going to call it fakery when, as already established a few pages earlier, he knows about wizards, and is unsurprised by mentions of schools of magic.
Even the obvious possibility, that “magic” in thieves’ jargon is used to refer to legedermain rather than actual magic, falls apart under both the later lines in that scene and his lack of confusion in the previous one.
One might also imagine that something similar is a frequent trick performed by con artists. While plausible, it seems a stretch to fit this to Briar’s thought process in this scene.
All that said, I’m still not sure why
Overall, I can’t think of any plausible reason for Briar to think this way. It’s not just shift over books, though, because the commonality of magic, and Briar’s very familiarity with it, was established many, many times
in this very book, both before and after this scene.
The best possibility I can think of is that at one time in the writing process, this might have been a very different book. Perhaps Pierce was originally writing a book where familiarity was less common, and magic was more secret or much rarer. Perhaps she even started from that very scene—four very different people in a dormitory at a temple of magic.
Perhaps, over the course of expansion and revision, the world became something different from what Pierce had originally planned, but she (or her editor) simply didn’t catch this vestige of the old story.