It's even more insane than that. Starke has been sitting on this info since the first 6 months of development. Considering they have been around for about 3 years and a half it means he mapped 6 years of progress and there's no guarantees the other games aren't planned as well, he just released this info now but apparently a few people already had it.
I found this on Google (major spoilers!)
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It's on the first page of results if you search "Fleeting Iris".
This is what I believe to be a leak with a small resume of how the routes will play out. There were some differences but it's over 85% correct. Here's the crazy part, the date is November 2017.
Were it another developer I would definitely think they were insanely optimistic but considering their deliver for the past 3 years I'm just going to go out on a limb and say their level of motivation is borderline insane.
I don't even know what I'm going to eat for dinner.