It seems to be a widespread misconception amongst commercial VPN users that port forwarding is required for torrents to work. While port forwarding can be beneficial in certain situations, as you said, it's certainly not a requirement, especially for well seeded torrents like I'm sure the large majority of people are downloading.

You don't necessarily need port forwarding to torrent, but if everyone was behind a VPN without port forwarding the network wouldn't work.

For two peers to connect, at least one needs to be reachable by the other. Behind a VPN that requires port forwarding, so if you don't have it you rely entirely on peers that are reachable.