The [font] tag allows you to assign any font to your text and it works independent of the forum software. Basically, it doesn't matter if the specified font is installed on the server machine or in the supported fonts list, once you assign a font to your text using that tag those with that font installed on their system will be able to view it in the assigned font, otherwise it's displayed in the default forum font. The posts you've linked are all in fixedsys so if it's not displaying them with that font for you, it means the issue is on your end and you may want to reinstall the font, that should fix the problem.
Well the odd thing is, it worked before, and now it doesn't. Aside from Windows Updates (which could likely be the cause of this, I guess), I don't think anything on my system has changed since I could see Fixedsys
"You see what your knowledge tells you you're seeing. ... how, what you think the universe is, and how you react to that in everything you do, depends on what you know. And when that knowledge changes, for you, the universe changes. And that is as true for the whole of society as that is for the individual. We all are what we know, today. What we knew yesterday, was different; and so were we."
- James Burke, The Day the Universe Changed (1985)