You are not exactly right, but going in the right direction. Not having a forwarded port means you aren’t ‘connectable’ by peers as your firewall will block incoming traffic. What this means is that only ‘connectable’ peers will be able to connect to you (your client can reach out to them as their port is forwarded). You are however invisible to peers that also aren’t ‘connectable’. You might also experience some degradation in time to connect as people can’t reach out to you.

To sum it up:

• Not connectable =X= not connectable (impossible)
• Not connectable =>= connectable (degraded time to connect when seeding)
• Connectable === connectable (perfect)

In my language SOA is STD. Which made me pause for a sec too on the title.

Didn’t mullvad stop port forwarding? That ain’t great for seeding.