It's dual licensed in a way, but neither license is open source. The OSI messed up in not coming up with an answer to the SSPL, and now ambitious projects that would have traditionally gone with an open source license like AGPL are now foregoing open source entirely and just slapping a sustainable use license on it.

So yeah, you can use n8n for free, but that doesn't make it open source. It is a source available license.