Was Julian really sorry when he made Auggie stumble?

No, Julian purposely cut in front of Auggie, and his apology was disingenuous.

"Yeah, let's just go," agreed Charlotte, following Jack down the stairs to the next floor. I started to follow her, but Julian cut right in front of me, which actually made me stumble backward.
"Oops, sorry about that!" said Julian.
But I could tell from the way he looked at me that he wasn't really sorry at all.