No, actually you were right. In fact his math says exactly what you said. He then averaged that 10% resistance out across the other 90% of encounters, for whatever reason. Which in no way negated your point that within those 10% of encounters it doubles your damage.
It's a themantic feat, if you are making a strongly fire themed character, it saves you from sucking in some percentage of encounters. In 3e for example fire resistance was very common, and fire based prestige classes like the pyromancer were frequently mocked as useless due to this.
OTOH if you are usually fighting Frost Giants and White Dragons, maybe you spend the feat elsewhere.