Who made that definition?
Ok. Where I come from we call that a premise.
OK. Defining words is ok. But that´s not the problem here.
The problem is: Who or what determined that Summum Bonum includes "cannot lie"?
After all, we learned in the other thread that Summum Bonum doesn´t mean "cannot commit genocide" and means "can do anything, if it serves a greater purpose (which may be beyond our understanding). I´m not seeing how interpreting "Summum Bonum" as "can not lie" is consistent with this line of reasoning.
Except that "three sides" is a factual, concrete, easily veri- or falsifiable trait (a proper definition), whereas "Bonum" is an unspecific (and, as we hear, defined by God) value judgement. So what you have so far are two terms defining each other - which doesn´t allow for the epistemological progress of excluding anything that isn´t provided by that definition.
I think you need to decide whether you want to accept the "Bonum" in "Summum Bonum" to be whatever God does (in which case you can´t exclude anything), or whether you want to apply an exteriour standard in order to conclude what "Bonum" means, practically, concretely.