No, the law never read that "exposing a normally-lawfully-concealed firearm is unlawful", but another law did read that (outside of certain exceptions which are the subject of another discussion) the open-carrying of a firearm was indeed unlawful. If a normally-concealed firearm was "exposed", even if only for a second, it was seen as "openly-carried" for that period, and therefore unlawful. It is not possible for something to be "exposed" and "concealed" at the same time.
The passage of SB234, as mentioned, did indeed "de-criminalize" the "brief" exposure of a normally-lawfully-concealed firearm, as you noted. Also, as you noted, the definition of "brief" has not been laid out, and I am not aware of any court cases that are likely to set a precedent for determining it. I agree that one is likely coming.
As far as "more arrests being made" is concerned, I feel that this is due, at least in part, to many CWFL-holders "pushing the envelope" more, and also due to an influx of newly-licensed carriers who "rushed through" their licensing process in the years following the 2008 presidential race/election, and have not become as "refined" in their carry practices as more experienced armed persons have become.