"Properly implemented" is doing the heavy lifting in that sentence.
Four paragraphs down from your quote is this:
Their attacks ranged from integrity violations affecting specific, targeted user vaults to the complete compromise of all vaults within an organisation using the service. In most cases, the researchers were able to gain access to the passwords – and even make changes to them.
If E2EE were properly implemented, the above would be impossible.
Imagine recommending Stremio when Jellyfin exists (you can use torrentio with Jellyfin as well).