I can’t say whether it can be cleanly migrated to another app but there are a number of export options to get you either to plain Markdown or something else within reason. When using the export option, the resulting Markdown at lest gives relative paths to any included media.

I don’t think it’s fair to suggest it’s not using Markdown. It just has a wrapper around it to make it work better for the usual use case of being a digital workbook.

Screenshot from the Joplin app showing the Notebook selector panes and part of the body pane. Tags and several other section names are anonymized. The visible section is Technology with subheadings for Cheat sheets, Flows, Python, Rust, and Wiki. A context menu is open from the Technology sheet showing the Export menu has been chosen and the MD - Markdown option is highlighted to click.