"Totally regardless of the racial sensitivity issues, couldn't you use a slur that is not so easily mixed up with a common gardening tool?"

it's not a slur it's an expression.  It doesn't refer to anyone or anything in particular it just means calling something what it is.  When someone says "Let's call a spade a spade." it means speak plainly and don't dance around the issues.  So rather than calling someone a certified automobile mechanical engineer you just call them a mechanic.