Fushigidane is part Poison... The problem would be if she didn't think it was part Fire to begin with. And logically I'd assume she was surprised that it was Fire/Flying rather than Fire/Dragon.
Since there's no subtitles available yet, I feel that there are some clarifications to be made. Iris referred to Lizardon as a Dragon type Pokemon. Satoshi corrected Iris by saying 「リザードンはドラゴンタイプじゃないよ。」 (Rizaadon wa doragon taipu ja nai yo, Lizardon is not a Dragon Type). The PokeZukan said that Lizardon was 「ほのおタイプとひこうタイプ」 (honoo taipu to hikou taipu, Fire type and Flying Type), thus there's no reason to believe that Iris thought it was Dragon/Flying (or pure Dragon) rather than Fire/Dragon. There have been cases in the anime where characters don't address dual-type Pokemon as both of their types (Isitsubute being referred to as Rock by Takeshi instead of Rock/Ground, for example), she probably took Fire into consideration, but stressed the Dragon type part because she's crazy about dragons. And assuming Iris isn't stupid at making educated guesses, it probably was the case. That, and seeing as how "dragons" in Pokemon seem take every possible dragon trope and appearance into account (western, eastern, serpentine, dinosaurs = dragons, etc.), Iris seeing a foreign dragon-like Pokemon using Dragon type moves and assuming it's part Dragon isn't too stupid of a situation, especially when it isn't native to the region.
That being said, I'd love to see Iris's reaction to Gyarados being a Water/Flying type, with no good explanation from the PokeZukan other than it being based on an Eastern Dragon (which would break the fourth wall of the show, more than Koiking's official classification as a "Fish Pokemon").