The article says "It is thus common to see moderate swap space usage even when plenty of free memory is available", with an explanation above. So it sounds like it's not always true for FreeBSD either.
The notion there is that at some point in the past free memory was scarce, so the kernel swapped out some pages, and that swap space may still be in use long after the shortage is alleviated. FreeBSD won't swap anything out unless there's a shortage of free pages.