No, it's not. You claim that, but monsters are not setting. They are monsters that go into a setting. An adventure is only setting for the adventure and nothing more. We have 8 pages of setting.
Reigar are no more setting than the ships and flying speed. Both are used in the setting.
True, but you will find the Scarlet Brotherhood in the setting book where it describes the area they live in, goals, behaviors, etc. This is what I get from the 1e Greyhawk setting on the Scarlet Brotherhood.
"Ruler: HIs Peerless Serenity, the Father of Obedience(true name unknown)
Capital: Unknown, but rumored as a hidden city of splendor and magnificence
Population: 35000+
Demi-Humans: Doubtful
Humanoids: Probable
Resources: Rare woods, spices, gold, gems(I, III and IV)
It is said that an order of monastic religious militarists was founded long ago on the remote plateau south of the closed city of Kro Terlep. This order is purported to espouse the cause of the Suloise as the rightful rulers of all the Flanaess, claiming superiority of that race above all others, and embracing evil as the only hope of achieving its ends. Supposedly the Scarlet Brotherhod is the fruition of these aims, and now it controls the whole of the land from the Vast Swamp to the tip of the peninsula. Brothers of the Scarlet Sign are reportedly hiding as trusted advisors or henchmen in many courts and castles in the north, spying for their master and ready to strike. The thieves as its lower ring, assassins next, and then the smallest and highest ring of monks as superior. The leader of the thieves is called "Elder Cousin," that of the assassins is known as "Foster Uncle" - thus other thieves are entitled "cousins" and assassins "nephews." The temple and monastary of the Scarlet Brotherhood is supposedly a fortress and walled town uno itself, guarded by soldiers, humanoid legions which are being readied for future conquest, and monsters trained to serve the Brotherhood."
That's quite a bit more than "Wanders around space not trusting each other because they destroyed their homeworld and exist to make art and war." which is just lore for the monster, not setting.