His conflict started with the Ontario Human Rights Commission policy which clearly includes gender pronouns. That is often getting mixed up with when he also spoke out against C 16 later, with different arguments.

http://www.ohrc.on.ca/en/policy-preventing-discrimination-be...

I think you are right that just in C 16 there is nothing about compelled speech. My understanding of his position is that it's part of a slippery slope, and the interpretation of discrimination or hate speech could easily extend to using the wrong gender pronouns.