Why is metabolism of ethanol catabolism? Could it be also detoxification?
These are somewhat loosely defined terms, but I would say catabolism is any metabolic process that extracts energy from a substrate. Ethanol conversion to acetate (via alcohol dehydrogenase; see this schematic) is an oxidative pathway which extracts energy in the form of NADH, so yes, it is a catabolic pathway.

I guess you can also call this process detoxification, since it removes a (mildly) toxic metabolite. Anyway, I think the "classification" of a pathway is less important than understanding the actual biochemistry.