There's a difference between believability and realism. Compared to the current tone of the game, riding Ahriman and Goobbues is way too ridiculous to fit in with the rest - it's not believable. I guess that's what he means with cartoony.
Anyway, I actually like the beastmaster idea as well, that sounds reasonable enough. But something's telling me that it's not what they have in store for us at all.