I think, in this case, this is not so much gatekeeping as a different definition of what it means to be an engineer. Most anywhere outside of the United States shares that definition - one that requires one to have met certain obligations, both in terms of education and demonstration of abilities, but also of responsibility for the results of their efforts - that the US does not have. An engineer is responsible for what they have built in most places in a way that software engineers are not, and prompt engineers even less so. Those individuals (rightly) feel slighted that the term of art that applies to them does not hold the same weight in other circles, and that those individuals profit off of this difference in definition.

Sometimes the gate is kept because to pass beyond is to take on a responsibility that not everyone is fit or willing to bear.

Prompt engineer is derived from software engineer, which only is sometimes justified. It is basically diluting the word with increasingly less meaningful context. An engineer needs to be able to make guarantees and statements about what he engineers. You can do that with software if you are careful but the vast majority of software development is not engineering because engineering is too expensive and overkill most of the time.

But now that AI is in the mix and the results are purely stochastic, you basically threw out the entire meaning behind the word. The prompt engineer is unable to make guarantees and accurate statements about his prompts. He can't predict what the machine learning model is going to do, otherwise he wouldn't have asked.