I worked in TV casting for 15 years. It's ugly -- there's a reason I got out of it -- but what I describe is standard operating procedure. There's also some other points in nuTrek that point to the same kind of thinking -- Marina Sirtis being the only returning cast member to get shitty billing on "Picard", Blu Del Barrio and Ian Alexander starting as recurring despite being featured with all the regulars in the key art for seasons 3 & 4 respectively.
Do I know for sure that's what happened with Horak? No, and I'm not claiming to. All I'm saying is I've been behind-the-scenes on A TON of situations that, to the viewer, look exactly like this one, and in my experience 100% of the time the truth of the situation was exploitative business affairs people. There were always more benign explanations that were theoretically possible, but those were never the reality.