Obsidian appears non-FOSS, it isn’t!

Where did you get the idea that it is FOSS? The page on it links to their releases repo where they plainly state,

Obsidian is not open source software

Degooglers and privacy people like it because it's "good" proprietary software that is fully local, doesn't phone home, and doesn't lock in its users. That's all well and good but it doesn't make it FOSS.