The word “scientism” by its structure should refer to the belief system of a scientist, but it has been dishonestly appropriated by a bunch of religious philosophers to refer to a kind of worship of science as the only source of answers to anything – including questions of ethics and value (about which as yet science makes no claims).
A more honest name for that uncritical worship of science would have been “scienceism” and its followers could be called “scienceists” rather than scientists (who are practitioners of science, rather than believers in its exclusive power to answer all our questions). So I think that the wrong word was used deliberately in order to gain an advantage in arguments with scientists by subtly creating an impression that they all believe something that many do not.
[This may seem like an unfair claim, but I am disinclined to give the “scholars” who introduced the term much benefit of the doubt as they were just the kind of language specialists who, if not completely stupid, could not have failed to notice the problem with that word choice.]
C.S.Lewis (who was not the originator of that duplicitous naming game) was against scienceism for the very good reason that there are many questions we can ask which do not have empirically testable answers.
“What is ‘good’?” or even “What should I do next?” are questions to which the answers depend on the evaluation of competing claims, whose relative importance at any particular time may depend on many things from the community to the brain chemistry of the questioner, and for which we have no foreseeable means of definition and measurement.