I’ve never quite understood the force of the term. Wikipedia has it that the term “scientism” “is used to criticize a totalizing view of science as if it were capable of describing all reality and knowledge, or as if it were the only true way to acquire knowledge about reality and the nature of things”.
Perhaps the main question is what counts as “science”. Now, we may gain knowledge of ethical matters, or matters of mathematics and logic, without doing experiments in petri dishes. Also, we may gain knowledge of who murdered whom, or of where the closest grocery store is, without publishing peer-reviewed papers in prestigious journals. So in one sense, science isn’t the only game in town for gaining knowledge of how things are.
But one might well respond that by “science” one means something of a bit broader definition, encompassing all properly reasoned and tested thought. Then mathematics and logic are part of the sciences, so too are everyday observational techniques such as those used to determine guilt or innocence in a crime, or those used in our daily lives to figure out how, where and why. Ethical thought is one outlier; one may bring scientific or rational forces to bear in ethical reasoning, but given the is/ought gap, there will always need to be additional arguments that are not solely dependent upon descriptions of the way the world is. That said, those who typically use the term “scientism” in debates do not appear to simply be making a point about ethics. Instead they use the term when their proposed method of describing reality has been shown to be bankrupt.