Historically, yes. It was basically like translating an ncurses (semi-GUI command line thing) into a UI. There were attempts to move away from that model, but at least when I worked for SAP, most actual deployments were still text-to-UI. It was basically a way of slapping some varnish on what was fundamentally an old-school mainframe system.

It's the same today - S/4HANA is still accessible from the GUI and DIAG is still used. Web applications (which SAP is pushing customers towards) are freed from this, of course (they talk native http to the SAP application server).

Remember that back in the late 90s SAP had a native GUI that ran on Windows, OS/2 and Unix (Motif-based) and each had their own native controls (a native Windows listbox is implemented differently from a Motif one, for example). Developers would develop a UI in ABAP with platform-independent controls and that UI would be sent over the wire to the client as DIAG and the client would translate that into the native control and data, etc for the end user.

and experienced users can move very quickly through screens like a terminal but Fiori UI5 is the root of all evil

Agreed! SAP's GUI is generally terrible for occasional users (luckily most have now been moved to web-based applications, of varying levels of user friendliness), but in the hands of experienced back office staff it works well. Not pretty, but very functional and with shortcuts for everything.

at least DIAG is relatively small on the network, probably as a result

> the issue of listboxes only having the currently shown items loaded

I challenged this design decision when Fiori was introduced and the thing is that statistically the lists are either small or huge. When they're huge, they got touched in less than 10% cases, so by not loading the whole content you save a lot.