I don’t think you accurately read their comment.
They’re not spending years in development planning and creating new, exciting content. They’re holding back basic features from the beginning and selling them to you over time for many times the base game’s price.
Yes because it’s a new game, putting all the features they developed in the previous base game and years of DLC would take them a long time. This is how software development works. They don’t just have everything feature toggled off…
Many core mechanics are different in each game, and even if they kept the code identical for the DLC mechanics the bulk of the work is asset creation, balancing, etc.
There’s no universe in which you’re right