Well, as for those pesky tardigrades... they really need dissolved oxygen in the water for their life cycle.

So whether they specifically hitched a ride to Mars or not, is pretty irrelevant: they're likely dead, or in permanent desiccated hibernation, since I doubt there's a whole lot of dissolved oxygen in Martian water.

(Unless... maybe there's a native Mars bacterium species that is generating oxygen within Martian water, but we probably would have detected spikes in oxygen if that were the case.)

Now if we're talking about Earth bacteria on the other hand... then you've got some possibilities there--there's plenty of anaerobic species all over everything that comes from Earth, despite best decontamination efforts. Make it a halophile anaerobic bacterium species, and they just might be PERFECT for colonizing the Mars water table, if they managed to get deeper underground on Mars somehow (most likely on a drill bit, or drilling equipment from Earth?).

So ya, I'd agree: there's no avoiding Earth to Mars contamination: it's already begun and can't be helped.

In fact it likely began long before there were humans on Earth, through material-interchange between Earth and Mars that has likely been going on now for 4.5 billion years.