It's total BS and would and could only apply to <= 1984/85 JBJ, if at all. JBJ in the later 80s and especially 90s was totally able to hit countless different notes (I'm too lazy to count or think about how many notes there are in an octave right now and how many his stellar voice could cover back then).

But coming back to Jackie: I didn't notice that many off-pitch notes, to be honest. Yes, there were some, but IML especially sounded quite good, even on the higher parts. Or did you mean the verses?