End-to-end encryption means that they don't see your encrypted files at all, even if they want to. Importantly it means it is impossible by design to make mistakes like accidentally not checking passwords on login https://techcrunch.com/2011/06/20/dropbox-security-bug-made-... .
Transport security and at-rest security is very important (and is the best you can do if you want Dropbox's ability to access your files, e.g., so their servers can show your files in a web interface), but it's not the same sort of thing as end-to-end encryption.
Transport security and at-rest security is very important (and is the best you can do if you want Dropbox's ability to access your files, e.g., so their servers can show your files in a web interface), but it's not the same sort of thing as end-to-end encryption.