I worked with data acquisition systems around 1998. RS-232 was still king (though USB was quickly displacing it), using DB-9 connectors... I always assumed DB-25 had always been for printer's parallel connectors :D would be really confusing to get RS-232 with one of those for me.
I think the author doesn't mention there were other protocols as well, like RS-485 which had a much greater range in terms of distance (and I think it also used DB-9 though if you connected it to the wrong protocol, one of the sides would definitely go up in smoke :D ).
We had WYSE dumb-terminals with orange plasma displays in my high school, with an empty slot to upgrade them to smart terminals. The keyboards were very reliable. They saw a lot of use for "WordPerfect 1.0 for UNIX".
The most unfortunate thing was that if you didn't stagger logins, the poor little minicomputer they talked to would slow to a crawl. Try getting 40 people to sign in at once, and the mini would became unresponsive for all or most of a 45 minute class period. But groups of five, and you could get ~50 people signed-in in few minutes.
Actually before the IBM PC the parallel printers used 36-pin (Centronics) connectors and the serial RS-232 interfaces used 25-pin DB-25 connectors.
IBM decided to use cheaper and smaller connectors than in the standards, so they replaced DB-25 with DB-9 (in PC/AT) and the 36-pin printer connector with a DB-25 with inverted gender, to avoid confusion with RS-232.
Due to the importance of the IBM PC, these smaller connectors have become de facto standards.
Dell had a number of personal computers around the 95'ish era that power supplies with an ATX compatible connector. But it wasn't electrically compatible so if you swapped a regular PC power supply or vice versa you'd like the smoke out of a motherboard.
I think the author doesn't mention there were other protocols as well, like RS-485 which had a much greater range in terms of distance (and I think it also used DB-9 though if you connected it to the wrong protocol, one of the sides would definitely go up in smoke :D ).
Anyway, very interesting post!