Someone can believe in free speech while choosing to censor their own platform - free speech doesn't mean every venue, platform and publisher must proliferate all speech. And as far as advocacy goes, I didn't see any mention of it on David Duke's Wikipedia page. Although nowadays every racist and anti-semite is a self-taught Constitutional scholar and friend of Voltaire. But advocacy is orthogonal to political belief, and plenty of people advocate free speech as a proxy for their own unspoken views.

Ben Franklin certainly believed in free speech, but he also wouldn't publish content he considered libelous and slanderous in his newspaper, rather he would tell people to take such stories elsewhere.