In-universe, this is the first time the inquisitors appear chronologically. So if someone's reading this comic, they wouldn't need to have seen Rebels in order to understand what's going on with them. True, most comic readers are not just casual fans as you put it, but there's probably a lot of people that have only seen the movies prior to reading this comic. You do have a point with Lords of the Sith though, and maybe that depends on the publisher/editors as to how much and what references are to be included. Why Ahsoka wasn't in one of the visions is up to the author and the editors at Marvel. The point is: Would it have been nice if Ahsoka appeared in a vision to Vader? Yes. Were the visions in the published comic nice, and did they serve their purpose? Yes. So, mission accomplished.