“I can’t believe you put these two thoughts back to back. Either you know or you don’t know. If you don’t know — as you indicate here — then how do you know the case cannot be won?”
Oh, for pete’s sake, that’s just a manner of speaking. You could say that the phrase “I’m pretty sure” is a rhetorical device. Certainty usually makes for more powerful writing, but no one wants to look like they’re dictating from on high about matters high and low.