The concept of... concepts has never been explored in Dragon Ball lore before or after the "Future" Trunks Saga. For Gowasu to, all of a sudden, be an expert on a subject that the franchise itself has yet to introduce or explore is nonsensical.
Like, the entire plot revolves around Goku, Vegeta, and Trunks attempting to prevent Zamasu from carrying out his ultimate "justice"—the Zero Mortal Plan. The plot has revolved around Zamasu's skewed sense of justice and self-righteousness. Why now is "justice" being used to describe the concept of justice, with Zamasu literally transforming into the concept, and not Zamasu's goals of molding the universe in his image? Not only does that fit narratively, but, hey, Occam's razor—to claim that Zamasu is becoming the literal concept of justice itself, a concept that has yet to exist up until that point and has never established beyond that singular line of dialogue, requires far more assumptions and logical gymnastics than claiming that, by merging with the universe, Zamasu has effectively become the ultimate judge, jury, and executor, capable of exacting his self-perceived justice without any resistance.
Besides, the Supreme Kai are the Gods of Creation, breathing essence into celestial bodies to populate the universe with intelligent life, and keeping a watchful eye on the cosmos of the lower realms. Their role in the universe as "gods" is purely cosmological, and has nothing to do with abstract concepts and the mechanics of space, time, and the beyond. Mainly because Dragon Ball has never explored abstract concepts and the mechanics of space, time, and the beyond... beyond the laws of time travel. The Kai are gods as well, and they have the same jobs as the Supreme Kai, but on a limited scale. Is King Kai an expert on abstract concepts, too?