I'm confused that Windows XP is the "Modern OS", and I want to know why there is no mention of any kind of linux.

I suppose it's to do with the drivers. Genuine question - why does it matter?

If the goal is get some hardware up and running with an OS, I've found it easier to get drivers for Windows OS, even older versions, compared to Linux. And that includes the more recent versions of Linux.

XP is an 18 year old OS that hit end of life in 2014. It isn't modern.

It wouldn’t surprise me if installing the latest NetBSD would Just Work on this hardware, SMP and all.

No part of this title is correct. Except possibly the "at home" part.

Opened the link to find 6x Pentium Pro. I think this person has never used a mainframe to know what its I/O capacity is like.