I enjoyed the article. It was a nice trip down memory lane. Regarding development tools, there were 2 commercial IDEs based on REXX: Watcom VX-REXX and VisPro REXX. I used Watcom's VX-REXX and it was a joy to use and allowed for incredibly fast and powerful application development. I heard the same about VisPro REXX. IBM's early set of tools C/2 and C Set++ were a bit painful to use. VisualAge C++ 3.0 was a decent toolset once you got over the weirdness of it. For a while, if you wanted to write C or C++ code using sockets you had to purchase IBM's TCP/IP SDK for $100.
The SIQ was a "synchronous" input queue and the problem has been understated in the article and comments. It was really bad. The base OS was incredibly stable, but the GUI shell, not so much due to the SIQ problem.
There were a number of Unix and Unix-like systems in addition to the ones already listed: Coherent, Interactive, and SCO are some that come to mind. They were pretty expensive IIRC, around $1000 to license.