It took an even closer look at the motherboard and discovered _all_ connectors are 4-pin. Inside the computer, 3/5 fans are 4-pin and the two on the CPU cooling tower (which are 4-pin, so it checks out) even have "PWM" in their sticker text.
If the BIOS/UEFI setting is already PWM, and the connectors are 4-pin, and some of the fans are, what's missing? And this makes it is even more difficult to understand why pwmconfig(8) asks for 3-pin fans. Because I have them - as well! fan front low be quiet! Shadow Wings 2 140 mm, 3-pin [1] front high be quiet! Shadow Wings 2 140 mm, 4-pin CPU cooling tower be quiet! Pure Wings 2 120 mm, 4-pin (2) [2] rear Corsair 120 mm, 3-pin projector extra fractal Silent Series R3 140 mm, 3-pin [3] (motherboard fan connectors are: CPU_FAN, CPU_OPT, SHA_FAN1, SHA_FAN2, SHA_FAN3 - all 4-pin) [1] https://www.bequiet.com/en/casefans/shadow-wings-2/1620 "available with or without PWM control" [2] https://www.bequiet.com/en/casefans/449 "Fan speed @ 100% PWM / 12V: 1500 RPM" [3] https://www.fractal-design.com/products/fans/silent/silent-series-r3-140mm/black/ (Doesn't mention PWM.) https://dataswamp.org/~incal/ebchw/COMPUTER -- underground experts united https://dataswamp.org/~incal