Um... It was basically reflex telling her to flip the coin. Tanjiro became a catalyst for her development, yes, but Kanao is her own character, not just some love interest. Remember that she joined the Final Selection of her own will and basically what Tanjiro did for her was+ https://twitter.com/knyconfessions/status/1310150430018633728

help her open up rather than help her have her own will, because just as Tanjiro said, the voice in her heart was small. He didn't say Kanao didn't have a will of her own, it's just that she's not listening to her heart. Plus, even Kanao said that it doesn't matter in the end+

telling of the trauma that she suffered from her parents, which made the voice in her heart "small". Tanjiro just pushed her to listen to it rather than push it down.

In short, Kanao has one of the best character developments even if she's not always on the spotlight.