Ha ha. Yes, guys, I have a dictionary too. My main point was the first sentence: it all depends on the wording of the sign. I wasn't attempting to provide a complete definition of "lesson". If the sign referred only to "lessons" and you (or a court) were trying to decide whether a "lesson" had been given in any particular instance, I think you would have to go beyond the dictionary definition. Context is important. The dictionary definition of a "lesson" as a "piece of instruction" is insufficient if offering a few "quick tips" is okay; arguably, that would be a "lesson" according to the dictionary. In the end, you'd consider a range of factors, including the nature and extent of the teaching, the expertise and qualifications of the "teacher", the nature of the relationship between the "teacher" and the "student", and (I think) whether the "teacher" received any reward for the teaching.
Not all of these things would be relevant in any given instance. For example, if I'd left my stall and was offering specific advice to another golfer for half an hour, I'd probably be giving a "lesson", regardless of whether I was being paid. Equally, however, I'd be giving a "lesson" if I was being paid for advice that comprised only a few quick tips given over a couple of minutes.