To add to the credibility of how Iroh became a great tea maker i'll remind everyone of the episode where Iroh accidently makes poisonous tea.
And if i remember right i think he also mentioned he knew the cure as well, and the question arises, why would a casual tea drinker make poisonous tea or even have the opportunity to?
How would he know the cure?
Deceiving appearences
It seems from all the episodes of avatar i've watched, and what i've seen of Iroh, i've learned that looks can be decieving. He appears to be a normal man who regularly drinks tea but when you follow the story deeply you find that even little things like tea become important.
Just remember that one board game he always played, pao something. Remember how the game tied him to the secret order of the white lotus?
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Sum it all up
1) He experiments with exotic teas (some even being poisonous)
2) Small items in his life become very important (board game and order of the white lotus)
3) He's known to be deeper than what is just on the surface. (Redirecting lightning, become super bulky in his jail cell, defeating the last dragons on earth but not killing them)
Iroh is definitely a tea connoisseur and he's definitely not to be judged by a first glance.