The ARS is slow and antiquated. In the US, and those that have bred for US releases, have a few models for what a grandiflora is.

The original model is the tall one, as described. Queen Elizabeth would be typical of this. "Stately," as they say.

Then, there are the quite round ones with large sprays and large blooms. Tournament of Roses and Sunshine Daydream would be typical of this. .

And then there are the "flori-tea" types, that have a small cluster of moderate-sized blooms on a medium height plant. Sonia, Caribbean, etc. are this type.

Of course, there is overlap and confusion. Its a really faulty system of classifications imo. Although, it seems that most breeders simply consider a grandiflora to be a bush rose with 3.5 to 4.25 inches in width.

Roses like Silver Jubilee and Colorific break the system. Not to mention many, many florist roses.

IMO, I would consider Sunset Horizon a large-flowered floribunda. It behaves that way in my garden, at least. Playboy has similarly large blooms, despite having few petals. In another system, we could be less subjective about the topic.

Edit: This rose is next on the winter pruning list, and given the silhouette that I need to prune --I can say two things. This rose has the final mature architecture of a grandiflora, and it is by no means short. In fact, the plant is taller, and slightly wider, than my much older 'Gold Struck'. I like 'Sunset Horizon' quite a bit, but I wouldnt lump it in with floribundas -- even the large-flowered types like Playboy, Blueberry Hill, or Daybreaker. The spray architecture is very wide, upright, and disjointed. The clusters that happen in those wide, disjointed sprays are in smaller sprays, often stem-on-stem within the massively wide sprays. I guess I can say 3 things about this rose. The third being that this rose is a stem-on-stem addict. Fortunately, its mostly upward.