Scientifics consensus supports the idea that living organisms require oxygen. then, a scientist discovers a type of bacteria that metabolizes sulfur instead of oxygen, and thus survives in environments without oxygen. the scientist concludes that a diverse group of many different organisms could live in an oxygen-free enviroment. why should are some scientists skeptical of the scientist's results and conclusion?

Couple of things:
1: only one type of bacteria was discovered metabolising sulphur, this does not necessarily mean that other bacteria, or indeed another organism can also metabolise sulphur. For his theory to be correct, many more and different types of organisms, metabolising sulphur needs to be discovered.

Secondly the use of the word diverse infers that more than just bacteria can metabolise sulphur. Bacteria are unicellular (single-celled) organisms, we therefore cannot say that multicellular organism can also do this, it could be specific to that one type of bacteria.