To put it the other way around, while the one containing soba can be grilled well to some degree by anybody, those who can grill the one containing udon well are very good cooks and so you can guess the level of cooking skills of the okonomiyaki shops to some degree by ordering the one that includes udon.
裏を返せば、誰が作ってもある程度上手く焼けるそば入りに対し、うどん入りを上手く焼ける者はお好み焼き作りの腕が立つということであり、うどん入りを頼むことでお好み焼き店のレベルをある程度推し量ることが出来る。