How did he make it clear?
Here's the issue with your situation...most people would be really stupid to actually go at a dog that is growling/barking at them. So in 99.99% of situations, a dog that can bark at someone, will keep that person away. So until someone actually GOES AT this dog, with an intent to harm it, you won't see what the dog actually does.
What your situation describes is what most novices see, and what most people want to see. A dog that comes forward, barks, does something to scare away a person. No one is really going to test to see if that dog will bite or not…because if it will, well, that’s kind of painful. There are other signs in the dog’s body language, that I can almost guarantee you missed which would’ve told someone that “knows” what the dog’s true intentions were. Those signs are very small, very particular, and not something a novice that hasn’t seen countless dogs either present or not present those things would be able to notice…especially in a high stress situation with a dog coming at them.
I’m not saying the dog wasn’t a good guard dog, but it’s almost impossible to tell from a story over the internet, and it’s almost as hard from just seeing him react to a neutral/friendly person. You weren’t a real threat, you didn’t present a threat after the dog reacted, so the dog won without having to take it to the next level. It is unknown what his reaction would be, if you did take it to that level.