OP needs to train their dog to not react to things not happening on their property or not have the dog outside. Not to mention what if someone does work on OPs property? Will they use that as an opportunity to train the dog without informing that person as well? They can easily end up hurting someone and having their dog put down.
To be honest, if you’re letting your clearly aggressive and large dog lunge toward and bark at someone, the threatening response isn’t unwarranted. The person genuinely felt he was unsafe or like this dog could come attack him while he was just trying to do his work, and OP clearly made no efforts to stop this from happening like taking the dog inside. You don’t know how secure that leash is, and if the dog gets off of it then it can easily harm the worker very quickly. And if it did, there’d be nothing wrong with him defending himself. If anything, his warning was courteous - OP couldn’t bother warning him or asking if it was okay to use him as a prop in training his dog.