IIRC, at the time of the original line's writing, the idea was for humans (Reclaimers) and Forerunners to be essentially the same species. I figured a Forerunner had asked the question, and over the years, or perhaps as a result of Guilty Spark's inherent mindset, Forerunner and Reclaimer had blended together and he confused the Chief with whoever asked it before - or perhaps he was using a general "you" and referring to Forerunners/Reclaimers as a whole.

Of course, we now have the whole "geas" thing (still unnecessary space magic IMO), plus humans and Forerunners being separate, so Guilty Spark may have detected the Forerunner personality in the geas and assumed the Chief was a Forerunner.