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.