Soma’s always been wily, but he also may be a bit more traditionally clever than people give him credit for. Yes, he’s hot-headedly hell-bent on creating his own specialty that embodies his cooking style, but that’s not the only thing behind his crafting of an appetizer that masquerades as a main dish. As he reveals in the final moments of the episode, what he’s really doing is forcing Erina out of her comfort zone. After tasting Tsukasa’s dish before finishing his own, Soma realized that even Erina’s “very good” just isn’t going to be enough – not only is she up against the First and Second Seats (when she was the Tenth), but she’s also got to convince her unfairly biased father of her food’s superiority. That means that she needs to go above and beyond, and she’s just competitive enough where Soma’s concerned that he’s the one to push her into it.
It’s an interesting statement on Erina herself as well as her cooking. She’s always been conceited, but that hasn’t translated into “competitive” in most cases. Her God Tongue has allowed her to wallow in her own perceived superiority whether or not that was true of her actual cooking.