Coincidentally (or maybe not, since the video was only posted on Youtube a week ago, so it's "current") I happened to watch that same TED talk post myself last night. The speaker could have used indefinite articles (an anti-weed guy, a pro-logic guy), but in context there's a subtle extra implication to his actual choice.
Saying the in such contexts more strongly suggests that both the speaker and his audience (and society at large, in fact) all share the same clear-cut image of what they suppose(d) the/a stereotypical "anti-weed" person to be like (i.e. - the archetypal example, not just one of many).
Since a big part of the speaker's message is the sterotypical image many of us share is wrong or misleading, it's very appropriate to "subliminally" call attention to that through his choice of article. He's a very good "persuasive" public speaker, and that subtle choice encourages his audience to feel that they and the speaker are "on the same side, thinking the same things".