Considering they've been around for the five major extinction events here on Earth, some of which involved very large impacts, it is almost certain that there are, to this day, bits of frozen, orbiting ejecta that actually do contain tardigrades.
Still viable? Almost certainly not. But it makes for a fun what if to ponder on the other option!
Or maybe tardigrades seeded Earth with our ancestors. Maybe tardis and tardigrade come from the same ancient language. If so, I for one am ready to serve our diminutive overlords.