He may well mean a mechanic qua mechanic is not capable or practising virtue -- not a mechanic qua human being.
The ancients tended to predicate formally where we predicate materially. When they said 'a mechanic can't practise virtue', they didn't mean every man who is a mechanic, which is what we would mean. They meant every mechanic in so far as he is a mechanic. At least this is the tendency -- I don't know if it applies to this particular case.
In this case, it would mean he can't practise virtue, at least not complete virtue, through being a mechanic, but he could in other areas of his life.
Jacques Maritain's Introduction to Philosophy explains this helpfully.
The ancients tended to predicate formally where we predicate materially. When they said 'a mechanic can't practise virtue', they didn't mean every man who is a mechanic, which is what we would mean. They meant every mechanic in so far as he is a mechanic. At least this is the tendency -- I don't know if it applies to this particular case.
In this case, it would mean he can't practise virtue, at least not complete virtue, through being a mechanic, but he could in other areas of his life.
Jacques Maritain's Introduction to Philosophy explains this helpfully.