If a notebook has sub-notebooks "folded" underneath it there will be an arrowhead to the left of the notebook name. Clicking on this will cause the sub-notebooks to "unfold".
When I switched to Joplin, Evernote wasn't able to save a sub-notebook tree in an .enex file. So it was necessary to export each notebook one by one from Evernote.
You appear to have all the notes from the sub-notebooks. Has the importer just lost the sub-notebook titles or have they become something else like a tag?
Thanks @dpoulton. I checked: and no additional tags were created during the import. So it looks like the Evernote Notebook Stack becomes a Joplin Notebook with all notes and no sub-notebooks when the ENEX file is imported.
I've been looking for an Evernote replacement ever since I made more than a dozen suggestions in the Evernote beta more than 3 years ago ... and none of them were taken up! Most of them were to add back features they removed...
Evernote 10 drives me up the wall: there are features I use every day (like creating a note from Outlook) that they didn't put in. As an Evernote Premium client since 2008, why should I pay for an app that removed features I used every day!