The statement above is technically true, though not in the sense that advocates of creationism typically use it. In reality, scientists use the word "theory" in a completely different context than the general public.
In science, a theory is a collective set of laws or hypotheses that explains a certain phenomenon. Instead of the highly uncertain, superficial guess that most of us associate with the term "theory," a scientific theory is typically supported by lots of evidence. While one of the qualities of a theory is that it's falsifiable (can be proven wrong), the notion that evolution is wrong because it's "just a theory" is incorrect. Albert Einstein's special theory of relativity is "just a theory" – but one that has been confirmed in numerous experiments.
