I'm not sure what this might be related to, perhaps because of the 1 project, the sequel is postponed, perhaps it will not just be an update for 2-3 chapters, but a full-fledged 2 part. Also, I can't even approximate the production time of this game since it is not an rpg or a renpi
It's just a story that was slightly revised with additional branches and had 3d art by 3rd party included, you can read it on AO3. A prolific author (or one that uses AI, like I have a feeling was used in this one - I've tested Perchance AI and several sections of this story reminded me of how it wrote) can have a published of similar length typed, proof-ed and published with a month. Hell, I've known people who pushed out 10k-20k word chapters out weekly and whose works end with several 100.000 if not over a million words (yes, they exist). Otoh there are people who struggle with 10k for 6 months or more.
The important part is to start writing immediately after finishing the prequel at the latest (if not simultaneously) cause if you wait too long you start to forget stuff you wanted to include, you forget who characters are, your plans for them, you forget nuances, what you want to say with specific turn of phrase, you forget to continue certain arcs, etc. Also, the more time passes from finishing the prequel the less you feel like writing a sequel. The adrenaline and drive from the original story lasts a week or maybe two, usually as long as the first wave of reviews lasts, then starts dropping along with motivation, especially if you get distracted by RL.
I've never created any AN's, but I was an amateur writer for a long time until I burned out (helped by a new job that had me on my feet and working for 12 hours a day, 6 days a week), so I know the problem of struggling with sequels personally.