As previously noted, it's not without precedent for Alton Towers to submit planning permission, or plans, with outdated information. A recent submission included The Log Flume and Black Hole on the general map.
Plans shift and change, it is likely that Horizon was going to be submitted for planning permission in 2020 or 2021, but COVID pushed plans back slightly. When Alton Towers eventually did get around to submitting the plans, it's likely that they missed this reference to the original planned construction start time.
I realise I'm trying to have my cake and eat it too, but I'm hungry. I'm also a goose. We like cake. 🪿