For me it is always, "talk about... what?" Talk about being a historical podcaster and what that entails? Yes, sure, great! Talk to a popular audience about some popular history? Eh, maybe. It'd be important to frame it right, but it's sort of the role that he himself tries to embody with his podcast, and there are limits to that mode of things, but then again, there are plenty of non-experts who talk about history and we've all sort of accepted that is fine, even when they make mistakes, so OK. Talk to a scholarly audience about historical scholarship? He's not qualified for that, sorry.

As an analogy, when I was a grad student, the science/speculative fiction author Neal Stephenson was invited to give a talk to my department. If he had talked about how he, a fiction writer without a PhD in history, did historical research for his historical fiction (like the Baroque Cycle) and the things that stuck out to him as being most interesting from the perspective of a "layman," and how he translated that into immensely beloved works of fiction, I think it would have been great. As it was, he tried to give a talk about the epistemology of the Royal Society of London, and the experts in the audience who actually had studied this topic for decades tore him to shreds, which was really awkward for everyone.

This isn't so much about "gatekeeping" as it is about "expertise," and someone like Carlin just doesn't have it. If you changed it to a "popular science podcaster with a bachelor's degree in some form of science," you'd get a similar sort of thing — there would be ways to frame that which would be acceptable, and there would be ways in which their expertise would be inadequate for the job.

Depends on the lecture, if it's media studies he's an excellent candidate to talk about creating a successful podcast.

As for a lecture on history: the past thread about how accurate he is (or lack thereof) when discussing history should be a good starting point for what historians would think about a history lecture of his.