I do suspect that some of that perceived dominance by medieval fantasy is heightened by labelling as such series and books which aren't based on the middle ages.
Conan, for example. Malazan. Codex Alera. Tigana. A lot of David Gemmell and Paul Kearney's work. Gentlemen Bastards, arguably. What is really meant by Medieval Fantasy, often, is 'pre-firearms' and there's an awful lot more of that than the opposite. But then, even if you do just count the middle ages themselves, they were approximately twice as long as the time we've had since then.
Even taking that into account though, I do think TC's wrong. It's still the dominant force in fantasy publishing, but it's hardly an overwhelming majority any more if it even is one. The medieval ones are usually the highest selling but the otherwise are getting out there and seeing more and more attention.