Bittorrent relies on your connection working both ways: to accept incoming and outgoing connections. Without port-forwarding: you may will see slower speeds, slower speed ramp up and if a torrent has very few online seeders you may run into a situation where you cannot connect to anybody at all - no download! Seeding is very hard without port-forwarding.
Normally home users cannot accept incoming connections due to NAT. This is always the case for proxies and VPNs, but some VPNs allow the assignment of a port that will always lead to your connected device (and to your client). Making outgoing connections is always technically possible but then you rely on the seed/peer to have their ports open! At least one side must be open. There’s no way around.
They even had a special FAQ-like section for users such as yourself right before the above quote.
But it has been running fine without any port-forwarding for me!
Can you explain the risk of port forwarding through a VPN?