I would expect that the metaphor refers to a mechanic’s ability to make logical, informed and accurate assessments and conclusions as to mechanical cause-and-effect, to have a no-nonsense approach to detecting and correcting problems in a straightforward manner, and to be in many respects “all business”. Perhaps you are perceived as brusque to the point of (or beyond the point of) rudeness. In that respect it may not have been meant as a compliment (not “complement”), but if it’s accurate then I’d accept that with pride… unless I were, for example, a hospice worker or a kindergarten teacher, perhaps. (And if I were perceived as rude, then I might want to address that, but that does not imply that I think mechanics in general or GM mechanics in particular are in any way rude as a class.)