Exactly, the issue is that no ticket # provided online (nor even "e-mail receipt" link - which is pointless anyway as even when it's there it usually doesn't work), if somehow the e-mail doesn't reach one and it's a partner airline booking it can be impossible to fly on it without either their confirmtion # (that DL does not provide anywhere, not online and not even on receipt!) or the DL ticket #. A less experienced person at the partner airline may not be able to find the reservation and may not be able to check you in at all.

What I don't get is why does DL claim to be a global airline, when it is so very extremely myopic and operates as some small regional southern airline (with Atlanta the obvious centre of the universe).

NWA was an actual global airline in how it operates. DL is a global airline, yes, but how it operates and (mis)represents itself is as a small, backwards regional carrier (in being very sloppy, zero attention to detail, poor IT, poor to non-existent support and treatment for non-US customers and partner airline bookings, etc.).