I also have a mellow guy who doesn't bark much.
When parents come to pick up and drop off their kids from our house, he often doesn't even get up and go to the door. He just lays there!
the day a man came to offer to trim our trees...this man was very persistent and would not leave...that time, my dog stood by my leg barking and growling at him the entire time. He actually acted so vicious, ears flat, showing teeth... that the man backed away off our front step and asked me to nervously hold my dog.
I think your dog will be able to sense when something is up.
Perhaps he takes his cue from you, and if you are a calm confident person then he in turn also feels that everything is ok.
I think he can sense that it is the pool guy, you were expecting him, you are calm and in control, this guy has been here before, etc.
When that tree-trimming man made me nervous, somehow my dog sensed it.
I think/hope that in a tough scary situation, he would have my back.
(But of course, it's a little difficult to actually test this, and I really hope that I never have to find out!)
Rumo ~ rescue shepherd/husky mix