You are right, but dude put in organic ammendment, so it can't be treated like hydro. The microbes and guano change the pH and will continue to do so. Plants produce exudates through their roots, and depending on what primary need for which element this will affect if the exudate is acidic or alkaline. Add this along with the influences from the organic additions. This explains the 6.5pH runoff. Which for him is what has kept his garden from really going to shit, despite the lower pH of the feed water.
Basically treat it now like it's an organic grow. Throw molasses in the water too to Rev up the microbiology and it help stabilize the root zone naturally and harmoniously.
My mix is mainly coco