I wouldn't necessarily say that it is viewed as a promotion.
Ken wanted to step away ... and so it probably made more sense for the QBs to see the game through Brian's eyes. Having the QBs more on the same page as the OC is generally a good thing.
Also, by NOT coaching the QBs previously ... Brian may not have appreciated the burden he was putting on his QBs and the tough job he was putting at the feet of Ken. By working explicitly with the QBs, Brian will have a much better idea of what his QBs know/understand and what they don't know/understand. That makes it a lot easier for him to simplify as needed ... which could help as it relates to the QBs "overthinking."
Of course, there will also be the question as to how much of learning curve there will be for Brian to teach QBs ... he clearly has the knowledge base ... however, the real key will be how well he can communicate and teach it (in an understandable fashion). He's shown that he can do so at other positions - so the teaching side of things might not require the biggest leap for him.