Honestly, I also thought it would be Steel-type instead of Grass. Goes better with its robotic look...
Also, Viperfox, sorry but you're totally wrong. A lot of pokémon are featured in the TCG predomantly with their secondary type instead of the primary. And that happens a lot when it comes to Bug and Dragon types. Examples: Victini, Tao Trio, Galvantula line, Ferrothorn line, Volcarona line, Forretress, Scizor, Vibrava and Flygon, Scolipede line, etc. (Some of them have had prints with their primary type, but most of their cards are printed with their secondary type). Or some are featured with their secondary type as much as their primary type like Lugia, Stunfisk (as you said), Celebi, the Krookodile line and more.
@DNA: the symbol on the promo Genesect is the CoroCoro symbol...