I should have been clearer; it's not just ebooks that are exempt, but anything without an ISBN. You can make a paper book with Amazon print-on-demand that doesn't have an ISBN, and then you don't need to send it to the dépôt-légal.

Many people argue this is bad and should be changed. But it's debatable.

According to https://www.bnf.fr/fr/depot-legal-pour-quels-documents, pure ebooks are not exempt; they are just archived directly from the distribution platforms by sampling.

What scares me is that some mixed works (PDF/on-demand prints) are not archived because the authors think the system is too clunky to deal with, which is really a shame for books like https://laurent.claessens-donadello.eu/pdf/lefrido.pdf, for example.

Are those books from real publishers or from Amazon's Print on Demand service?