I doubt that anyone takes creative briefs too seriously these days, so take this for what’s worth. “Straw man” refers to drafts or work-in-progress hypotheses that enable teams to get to increasingly better solutions. They are a tool that enable iterative process to move forward in informing and improving the final answer. I’ve always used them like that, which used to be pretty unorthodox. But it did create better results in the end.
Don’t worry about making your creative brief perfect. Make it useful.