There are a lot of people on this site that believe science is the only valid, or by far the most reliable method of determining the truth of reality. And they say so all the time. Which is 'scientism' by definition.
What we say is that empiricism is the ONLY path to knowledge about how the world including our bodies and minds works. That attitude can also be called scientism, which is neutral, nonjudgmental, and descriptive description: "Scientism is the opinion that science and the scientific method are the best or only way to render truth about the world and reality."
But as Blu mentioned, there is another way the word is used - the way you use it - which is meant demean and to disparage empiricism: "excessive belief in the power of scientific knowledge and techniques."
And yet they constantly deny that it's a thing even as they engage in it.
What I deny is that you or anybody else can derive knowledge any other way than empirically, although you might not share my definition of knowledge, which is the collection of demonstrably true ideas that can be successfully deployed in anticipating outcomes. People who use the word scientism disparagingly are people who want other ideas obtained by other methods called knowledge because they like the ideas even though they cannot be used for anything.
They mistake facts for truth and knowledge for wisdom. And that's dangerous.
What's dangerous is considering ideas that aren't demonstrably correct either truth or wisdom. Incidentally, knowledge is facts and wisdom is a subset of truth. The collection of facts one knows is his fund of knowledge, and the facts that help one find happiness are the subset I call wisdom. Thus, knowing how to make money is knowledge, but knowing the limits of what that money can do and the limits one should apply to oneself in obtaining it are wisdom.
The scientism are true believers in science as the only legitimate avenue of truth. It's why so many of them are also atheists. They can't tolerate any other possible avenues of truth.
Empiricists find other avenues in pursuit of truth sterile. You mentioned, "other means of acquiring wisdom, like philosophy, art, and religion." There is no wisdom in either art or religion. There may be beauty in art, or other things of value like community or reassurance in religion, but neither of those deserves to be called knowledge or wisdom. Knowledge in art is limited to knowing how to create it and knowing what the artist intended. After that, only aesthetics remains.
Visual art and music play a large part in my life and add to my pleasure living it. Wisdom is the knowledge that this is true, what art satisfies, and how to create or acquire it. But the art itself is neither knowledge nor wisdom. It's just beautiful or intriguing sights and sounds. Some people want their deeply cherished beliefs to be considered knowledge. They want their creationism, for example, to be viewed as truth and knowledge and want it taught in the institutions that exist to disseminate knowledge.
And those who make the distinction between ideas like that one and empirical knowledge like the theory of biological evolution and who disagree that they are equivalent are disparaged by those who want the same respect for their unfalsifiable and otiose beliefs as ideas that can be used to optimize lives object to ideas being rejected as truth, knowledge, or wisdom, and angrily begin writing and saying the word scientism.