Edit by 2020.08.18: The disclaimer is gone now; it’s officially production ready after all!
Original message (2020.04.11):
“Enterprise grade” maybe, but it’s certainly not production-ready, and I’m disappointed it was ever presented as such, let alone still. Anyway, it could previously be enabled by installing the app from within Nextcloud, but I don’t see that option now either. If you really want to use it despite the warnings, you should still be able to download it from github into your nextcloud/apps folder, and then enable it from inside Nextcloud.
The key feature here is that end-user’s devices are the only ones with the ability to decrypt the data - it’s encrypted on one end, sent to the server (which never gets the keys for it), then decrypted by the user’s device on the other end.
With TLS/SSL you have encryption from the device to the server, then from the server to the other device, but the server still has access to the data in the middle.