It was a genuine question, and while you reiterate the author's point about this being "bad security practice," neither you nor the author explain why this is the case.

I don't believe physical separation really buys you much here. At most, if may reduce downtime if you do indeed get pwned, but I think that you can achieve the same objective through a combination of containers, VMs, and UNIX users. And running multiple, somewhat redundant machines also has obvious downsides such as increased power consumption, increased maintenance burden, additional space and cabling, etc.