While Obsidian may not be FOSS at all, my understanding is that in general needing to purchase a license is orthogonal to whether something is FOSS - correct?
If you cannot use the software however you wish (including for profit) then it is not Free Software. The developers can choose to not distribute the software to you unless you pay, and they can choose to provide support if you pay (which if you're making a profit from their software is a reasonable thing to pay for), but with Free Software the developers cannot decide how you use it once they have distributed it to you.
However, this becomes less significant when the applications are interoperable. If you ever stop using Obsidian, you can just grab your files and use another Markdown editor on top.
