"Everything is interpretation."
- Zen Koan

(Actually I just made it up.)

You seem to be saying that you believe the article is claiming "new microbes prove life on Mars exists!" IF that were the case I can understand your reaction. But I don't believe you actually think that's what it's saying and I don't believe anyone else would take it to mean that either.

For your fish analogy IF there were OCEANS on Mars and it was believed these oceans were at subzero temperatures then yeah fish surviving on Earth at subfreezing temperatures would give credence to the idea that the same could happen in such Martian oceans.
But there are currently no oceans on Mars. There are areas on Mars with ice on the surface and near subsurface frozen water (from the Viking lander results.) What this research shows is that in itself is no impediment to life existing there.

Bob Clark