You are right when you say the signal is BOLD and not flow directly. Flow regulation is a key component though. However, I don't think this negates my point. Oxygenation is being equated with brain activity. The approach is sensitive to BOLD signal changes that correlate with the input. I'm not a neuroscientist but I know many that would argue the brain is more complex than this and response are more complicated (what about areas of the brain that are on through out the experiment?)