would probably have done something petty and vindictive at some point anyway,

Yeah, that's the basic reaction here from me too. It just isn't a response that you would EVER get from someone who was in a normal amount of touch with reality and emotional regulation and was capable of being a normal coworker with other adults and clients and professional relationships. And that's because it isn't a normal or proportionate response to literally anything! The slenderness of the excuse and the bizarrely disproportionate and immediate response from a complete stranger in whose place most people would be at pains to make a good impression makes it absolutely chilling.