I think Iroh knows true Firebending, he doesn't seem to rely on anger and evilness to firebend, Jeong Jeong doesn't rely on that either, but believes that fire is just destruction and pain, and I don't think Iroh views Firebending, plus he can defeat Azula who is a "Firebending Prodigy" in an Agni Kai because again, he doesn't rely on wickedness to bend, and he has WAY more experience than Azula in firebending.
