Thanks for all the answers.
On one hand, the assignment doesn't seem like something they could directly use in their business, mainly because it comes with data not from their industry.
On the other hand, even though during the interview we agreed on one assignment/task, the hiring manager later decided to send me two more. And he called me the next day during work to ask how I'm doing solving it and if I have any difficulties.
In the end I decided to decline and sent a very polite email - I don't think I would have enjoyed his management style in the long run