That's what the IFCC wants you to think. The allusion that the three made to a vessel and Sabine before the end of Utterly Dwarfed preceded the first Julia visit. If the IFCC is involved, and if Sabine is involved, at least Sabine knows what Julia looks like, and she can, posing as Julia-the-wizard-student, present to Roy her cool hack on Dad's blood curse. I suppose that there would need to be a way for the IFCC to know about Eugene's blood curse and the connection with Roy and the sword. I can't for the moment recall where it is shown that they know that, but Sabine might.
So it's possible, but for my satisfaction a few more pieces need to be shown before it settles. The mechanics of how multidimensional stone works remains fuzzy, but it might not need much of a loophole to wiggle through.
Another indicator of the first visit not being Julia is the "yeah, cool spell hack, but I can't teach it to your wizard for *reasons* complete with uh, er, stumbling by Julia) Strip 1192. The voice sounds more like Eugene in a few of those panels, but not in some others and not as much in 1193 ... and she does keep harping on Roy beating Xykon ... which is Eugene's obsession. One thing about Eugene is that he's a bit of a nag to Roy about the oath, and that he is, or was, an illusionist, and he knows Julia and Roy well. And he wasn't at Roy's fighter school test, I think.
This being another Eugene trick to keep nagging Roy about ending Xykon is a less complicated take if the Julia we see here isn't Julia.
The usual answer to stuff like that is "A wizard did it" and "it's magic, you really wouldn't understand" but Rich usually doesn't go for that.Julia apparently has the resources to scry Roy all around the clock, to learn when he has time to talk to her. And then, she just appears. I find it suspicious, Roy called it out many times before, but that was with Eugene. With Julia, I am highly sceptical she can do that. Especially within this labyrinth made out of multidimensional stone.
I presume that's the 'soon to be breakout character of the entire series' which puts the Chaotic Giaffes Hilgya summoned back in the unemployment line.