I’m guessing they meant this:

Germ-free organisms are multi-cellular organisms that have no microorganisms living in or on them

But I haven’t been able to find anything concrete about what this facility was. From my experience, they’re not creatures with “no microorganisms at all” but just “extremely controlled microorganisms” and germ free mice often lack, for example, things like gut fauna. It’s not a pleasant life for the mice.