Precisely. If you are playing a G-major blues, you cannot start start wanking around the G scale, as if anything in the scale fit at any time. Rather, you need to play G chord tones over the I chord, D over the IV chord, E over V chord--using appropriate passing tones between chords.
Yes, yes, I know that "The chords come from the scale. You have to know the scale to know the chords." True, and Carol wouldn't disagree. But when you are playing over the I chord in a G-major blues, pedaling a C sounds like ass. Yes, it is in the scale, but it is really only a passing tone from B to D.
That's what Carol Kaye means. And, it (like just about everything she says) is really good advice.