The app is not. That said, since it’s literally a front end for markdown files, the file format is universal and can be opened on anything with a text editor. No conversion or export needed.
I know some folks won’t use it if it’s not FOSS, and I respect that decision. For me there isn’t an open source alternative that is as good. Joplin stores your data in an SQL file, that’s a deal breaker for me. Logseq seems to be so outline/task oriented that I’ve not been able to use it the way I want to.