"Because the two of us have fought alongside demons before. I was mind controlled, so a lot of it is still blurry for me, but I remember."

Since she's had direct contact with him as Robin, and Cass doesn't make much of an effort to conceal who she is, it would only be a matter of time before she figured out who he was. Tim might as well get that out of the road, so they can focus on other things.