^Right. Science isn't about making absolute statements about everything that can ever exist anywhere. It's about making conclusions and predictions about what we can observe and measure. Although, granted, calling it a "habitable zone" instead of a "liquid water zone," say, is a bit misleading.
Then again, when they say "habitable," they don't mean habitable to humans, just potentially habitable for some kind of life as we know it, even microbes.