First of all, please note there are two ways of adding friends in Slowly: adding by matching and adding by Slowly ID.
Only adding friends by Slowly ID requires acceptance of friend requests. The pending requests can be found inside Friend Requests in Home screen (i.e. under Recently Received).
For other new friends added by matching, i.e. Manually Explore or Auto-match, you would directly receive the new letters from them - no need acceptance of friend requests.