I agree. However I think nothing will change for some fans who haven't convinced if Ash appears in Horizons himsef. They will say he is an AU Ash and continue to ignore Horizons and Ash Era are in the same universe.
Yeah. I'm also a bit astounded that the anime has not taken advantage of introducing alternative regional gym leaders, aside from the ones introduced in video games, that also exist within pokemon anime unviverse.
For example, in the original anime series and beyond, we are informed that there are multiple gyms within the region of Kanto, aside from the eight gyms featured in the video games, that are available for pokemon trainers to challenge.
If the anime wanted to avoid every previous gym introduced in the Ash-related stories, the writers could have got creative and introduced other original gyms that exist within those regions. For example, for a training mini-arc, the anime could have introduced / featured, in Liko and Roy's early story arc, multiple new gyms within in the Kanto region based on jurisdiction. The anime could have added, that the eight badges collected by Ash are found in the northern region of Kanto, while other kanto gym badges are available within neighboring regions; east, west and South of Kanto.
For marketing, the anime could made some of the new kanto gym's introduced in Liko and Roy's adventure exclusive in future games