Yeah in Diamond and Pearl, three of hid pokemon aren't fire type but they still know fire type moves and could be considered honorary fire types. Flint is more of a fire type trainer than Brock is a rock type trainer in Red/Blue because despite using all rock types, neither his Geodude or Onix know a rock type move.

Using 3 pokemon that aren't fire types but no fire type moves is genius, oblivious trainers will go into battle expecting there water, ground and rock types to do damage only to be surprised when most of his pokemon don't have the weaknesses fire types do.

His team in Platinum is fine but his diamond and pearl team is better.