This isn't to say it's good, just that it's not new.
The serial port serves dual-purpose. Along-side the obvious serial, you can also provide some inputs by grounding certain pins, and some outputs by sinking current from others. This has got me through some unorthodox integrations by having the UPS signal that it's on battery by having these "simple signals" close a contact/relay on the load. It's not quite a full suite of dry contacts, but it's pretty serviceable.
The problem is that 9 pins doesn't leave many spare, so to achieve this they've repurposed some. I really wish they'd implemented this as an alternate mode so you'd, for example short tx to rx to change the behaviour - instead of having the 'alternate behaviour' sitting on standard DTR/RTS pins.
Getting industrial interfaces at SOHO prices was awesome. The unintended consequences .. less so.
It works perfectly with apcupsd though, even showing up in my desktop power settings as a UPS.
That was before USB was available so they used regular COM port communication, I'm not surprised with that early foothold there was no reason to change and sufficient force to continue ever since.
At the time it seemed like the un-necessarily non-standard RS-232 cable was just to create a "profit center" from the cables themselves, along with the non-standard replacement batteries. They sure would have been a lot quicker to market and had lower up-front engineering costs if they had used standard batteries from the beginning.
It's good to recognize early when excessively high TCO is the primary feature around which a product (or company) is designed. There can be significant PR effort from the beginning to divert any perception of anti-consumer attitude.
Lots of stealth can be involved to muddy the comparison with alternatives which offer normal TCO or well-engineered low-target TCO.