Shen Shi wasn't being polite; he genuinely didn't need Yulan to imprint the Primordial Energy Steel Seal.
Actually, the main reason Gong Chu and Gong Mo imprinted their allegiance to the Primordial Energy Steel Seal was because, at the time, his strength was comparatively weak, and he didn't have enough power to protect himself in front of those two.
But now, it's entirely different.
Even if one day in the future, Yulan's attitude changed, Shen Shi was confident he could easily foresee it in advance, or even if he didn't notice it, Yulan alone probably couldn't cause him much harm. Of course, more importantly, this opening must not be created lightly.
