"illegitimate" criticism is intellectually dishonest or tactical (Israelis are Nazis, Jews would be safe/free in 'one state' solution, there was no Jewish Nakba etc.). Legitimate criticism is at least plausibly honest (Gaza war was mistake, settlements are illegal). That is the difference. dishonest/illegitimate criticism doesn't deserve to be engaged because it doesn't treat its audience with respect, candor. You don't debate liars.
Abbas: West Bank arson is a 'crime against humanity', vows to petition ICC (Haaretz)
from the article: For Israel, 'delegitimization' is becoming an excuse