Sure, if they believe it. For the sake of argument, I’m assuming that all these statements are made honestly.
“F lives matter” is obviously not a racist statement. You could conclude that someone is racist based on that only if you can show that no non-racist would ever say that. Which is highly unlikely to be the case.
Well–it might actually be the case in C’s particular circle. C and everyone like them wouldn’t be caught dead saying “F lives matter”, even if they believe it, because they know the inference everyone would make. So the belief about its validity turns into actual validity–but it only works for that community. C applying that logic to anyone else is invalid. B may simply have not gotten the message. Or, they may not care if C thinks they’re racist.