The friend acceptance in Slowly depends on how you matched with your friends, and not all connections require accepting a friend request. There are two ways for adding friends in Slowly: add by Slowly ID and add by Matchings.
Add by Slowly ID:
Adding friends through Slowly ID requires friend request acceptance. You can find these pending requests in the "Friend Requests" section on the Home screen ( in the bottom menu.)
Add by Matchings:
For friends added by matchings, including matching from Open Letters, Profiles, and Auto-match, you will receive their letters directly. There are no friend requests or acceptances needed. The incoming letters will be shown on the Home screen, indicated by a notification. You can also go to your friend list ( in the bottom menu) to see who your new friend is before the letter arrives.