It looks like it depends on the definition of open-source. It doesn’t use an open-source license, but the code is indeed publicly available (just not redistributable).

https://forum.obsidian.md/t/is-it-true-that-obsidian-is-already-open-source/46413

Regardless, I may have to give it a shot for personal use - the nostr writer feature looks awesome! Thanks for sharing!