There are a lot of misconceptions currently held by non-technical and semi-technical people with respect to HTML5, what it is, what it's capabilities are, how well-supported it is among various web browsers, and how it relates to "standard" HTML. So on the one hand I'd fully support any effort you wanted to make to help clear those up.
On the other hand, in terms of applying for a job that may not be the best thing to do. If you can do it well and if your interviewer is a clever and reasonable kind of person, then you can certainly earn a lot of points for yourself by showing that you understand the technology better than your interviewer. However, if those things don't hold true then contradicting your interviewer's understanding of something might just upset them and/or cause them to think that you don't know what you're talking about.
So there's a fine line to tread. You can play it safe and just stick to answering their questions directly and demonstrating what knowledge you do have about HTML5, and explaining why you haven't delved into it more deeply (if necessary). Or you can take a gamble and try to explain to them how their understanding of HTML5 isn't really accurate/applicable in a real-world context. If you do that well (and if your interviewer is smart) then you can make yourself look very strong. Do it poorly (or get a bad interviewer) and you've basically killed your chances at getting the job.
Also note that when dealing with an external recruitment agency it's generally unlikely that the person you're talking to will have much in the way of technical knowledge or background, or much reason to care about learning new things about a technology. It's more likely that they'll just see that the requirements say "HTML5" and then consider only candidates who claim to have strong HTML5 skills. I think there'd be little sense in trying to explain to the recruiter that their understanding of HTML5 isn't really correct or that the job doesn't require HTML5. There's just no reason for them to care. They were hired to find someone with HTML5 skills, and would almost certainly just dismiss any candidate who claimed that HTML5 was not required.