[...] So... why can't there be different species of 'ID', arguing for different designers who designed different things? Why can't life have one kind of designer (patron races in Brin's universe) while cosmology has a different designer, assuming that cosmology has a designer at all? The whole idea that all the design arguments have to converge on one single Grand Designer seems to be a monotheist assumption built into the design arguments. [...]
That can be valid if taking the expression "intelligent design" at face value without prior commitments in terms of definition; or outright creating a general category likewise called ID for subsuming specific examples of the concept under. But usually it's the narrow "covert creationism" version of ID (which apparently combines cosmology and biology, if going by FAQs and papers of some of its "scientist" backers) that receives the attention from both critics and proponents. And thereby gets treated as if having property rights for the label.
Intelligent design arguably re-entered public discourse as a force in the late 20th century via Charles Thaxton's publications, but it's winding-road ancestry takes it back to the teleological argument of Medieval times and antiquity. Which variously had a similar aura of either Abrahamic theism or the earlier "creator-ish" thoughts of the Greek / Socratic era surrounding it. Thus that ancestry may also contribute to the bias which makes center-stage that particular species of ID which the Discovery Institute and any other think-tanks advocate (while trying to mask its religious provenance).