I agree. The amount of fuel in the aircraft is measured in weight, but the amount of additive to be added is in volume.
The AMM writers were a bit sloppy with that procedure. I haven't checked the Airnav glossary recently, but I wonder if PPM is actually there. And in any case, how hard would it have been to clarify and write Parts Per Million?
Airbus manuals are usually full of charts and tables. Why didn't they add one detailing the amount of additive according to fuel tanks contents.