It's unfortunate that the other users are ignoring your actual question... You should still be able to bind qbittorrent to the wireguard interface, and you definitely MUST do so in order to make sure you're safe (if the VPN drops, you don't want it to fall back on your normal connection). If you aren't sure what the wireguard interface is names, try running ip a before and after activating the VPN connection and compare them.
Port forwarding allows other users to connect directly to your torrent client. Without it, it's much more difficult for you to connect to other people who aren't port forwarded (though not impossible if there's a third, mutually connected client who can facilitate initiating the connection). Things will generally still work without it, but youll connect to fewer people, so it might be slower. And if you're downloading rare torrents, you might have to be patient and wait for someone else to join and facilitate the connection