There doesn't seem to be a word in English that captures the sense that you are looking for.
One who uses the methods of science is called
One who places dogmatic faith in a system might be called
a dogmatist or dogmatic
Methods of science are usually synonymous with rationality, which includes the avoidance of dogma. But that doesn't preclude people from acting dogmatic about whatever belief system they have. Dogma means a set of rules that you must follow. The methods of science are methods that were found through experimentation and comparison with a common reality; some may learn those methods as rules and follow them dogmatically.
'Scientism' sounds to me like a variant of theosophy, Christian Science, or even scientology, and doesn't associate with dogmatic support of scientific methods.
So a single word doesn't work, but there are multi-word phrases or neologisms that might work:
dogmatic scientist, hyperrealist, hyperrational.
all of which give a negative light to the scientific part which is what you are looking for.