As a past user of print brightener,( Nova and Fotospeed's) I has always assumed that the claim of an extra 10% brightness applies to the developed colours only but maybe not. If it works on the borders as well then it might be an alternative to anti-foggant which if I have understood PE correctly needs 3 anti-foggants( one for each colour).

PE understandably gets tired repeating himself but he may chime and say if these anti-foggants can be purchased. I suppose that even if they can then unless they were relatively cheap then it's a choice of putting up with slightly creamy borders or buying new stock. If inferior but cheap stock plus antifoggants = the price of new stock then it's a no-brainer