It's more like a , you might get killed by a car if you step into the street kind of theoretical. Or the drip of a water clock will fill the container in 8 hours kind of theoretical. Or a don't plant too early or your people will starve kind of theoretical.

A theory in science is a model backed by the huge majority of observational evidence and the collective experience of the model's community. There are no other equivalent human institutions for reliability. None.

So saying "it's just a theory" in the case of uranium-dating is a fairly unreasoned attitude.