I wouldn't worry too much about the topics: https://news.ycombinator.com/item?id=27666763

To add to that, topic modeling/content-based filtering is only used as a starting point anyway; as you rate the newsletters you receive, your recommendations will start to be dominated more by collaborative filtering ("people who liked X also liked Y", without thought for what X and Y are about).

Also, the "21 days" thing is pure marketing/explanation :). It's just an attempt to help people (especially non-technical people) understand that it'll adapt to your preferences over time. It'll start adapting immediately, and it'll continue adapting after 21 days.