Chose which player is activated by headphones play/pause buttons
Bluetooth connect &amp; Play from Play Store meets your needs. App description says

This app is designed to tie the Bluetooth connect event to playing the music player of your choice.  Unlike other auto-play options, this allows you to select the specific player that should be used.

I have Jabra Elite 65T (not active) and it worked by letting me choose which music player to be activated on key press. I didn't try with your audio book player, though, but I guess it should work.
Bonus point is that it has zero trackers as per Exodus