When you pass by another car do you raise your hand or roll down the window and yell hi? When you walkl past someone on a busy street do you say hi? Is there some written or assumed policy or cycling rule that says you must recognise someone else?
People are just people whether they are on a bike, walking or on roller blades. I have had this happen to me plenty of times and actually have done this at times myself. The lst time I had just topped a long hill and just didn't have extra energy at the time to lift a finger. Next time you see this biker just say hi.