He is being called a transphobe because he wants English to contain a new word which is singular and non gendered because he doesn't think 'they' is appropriate since it has historically been collective. Since he uses his new invented word for this purpose, he is being called transphobic for not respecting peoples choice to use 'they'.
When you look in to the details of what rms is being grilled over, he looks like someone who is honest and well intentioned but fails to communicate in a non controversial way.