It is my understanding that it is the former, not the latter. This way they can add content into different pieces without it affecting the overall story (and coding). Netorare (NTR) Route has to be added into every part of the game already created, as well as every part going forward. Then the Sharing/Friendship Route has to be added into every part of the game past, present, and future.
From the Patreon post last week
#30,226:
Whilst we will be looking into the possibility of making one complete build,
due to the ever-increasing size and complexity of the project, we’re breaking the game down into various seasons to try help manage this.