You quoted it yourself: scientism is "excessive", or, more correctly, it treats scientists as authoritative sources of truths that should not be questioned. This form of blind trust goes against the scientific method.

Also scientism can make categorical error in the type of questions that can be answered e.g. journalists asking physicists about "god particles" and such

> I'm struggling a bit to understand how a belief in the importance of scientific knowledge and techniques equates to a religious mindset.

I think the poster refers to dogmatic mindset. Assuming that all religions are dogmatic is incorrect.