That's absolutly true, but when a flight is "payload optimized", you are only 'protected'/included in the basic weight if you have it reserved 'early' (i think it's either 40 or 45 mintues prior to departure). Said another way, unless it's changed, you aren't protected on the JS on a payload optimized flight if you reserve the JS 16 mintues prior, for example. Basically, once boarding starts on a PO flight, they cutoff that protecion.

Glad to be corrected with the correct time, or if something has changed in that. (I've been burned by it, which is the only reason I know, but that was at least 6 years ago.)