I've noticed that some kindle-oriented features on the Amazon site don't work on my 5th Gen paperwhite. I assume this is because the paperwhite doesn't run the Kindle app. Maybe it's related to DRM, I don't know. But when something on the site says "buy for kindle", sometimes it means the Kindle app, and it's not intended for my paperwhite.
What about Kindle Unlimited? Can I really download Harry Potter books onto my 5th Gen paperwhite with a Kindle Unlimited subscription? The landing page says "any device", and I don't see anything ruling out the paperwhite.