Most species can be sexed the day they hatch, and in many cases that's the easiest time to do it (it depends on the species, method and skill of the sexer).
Many people are completely terrible at it, others are very good. Some will blatantly lie about the sexes (especially selling males as females since females are usually more sought after). Many vets have absolutely no idea what they're doing, but people believe them because they charge money so there's an expectation of competency. Most reptile specialist vets are reasonably competent with sexing, but certainly not all. Some people are willing to say "I don't know" or "I think it's x but I'm not sure" and others are not and will always give a confident-sounding answer even if they don't know. Some of these people are liars and some of them believe their own bovine waste.
Obviously in your case one or both of the people sexing the snakes had no idea what they were doing. Chondros aside, most snakes are easily sexed as babies, though not everyone is willing and able to do it (and there are plenty of people willing but not able, and also able but not willing).