From the article "Why Transphobia is a Feminist Issue" linked above:
"This is a feminist issue on two levels. First, like racism and homophobia, transphobia grows out of and helps perpetuate the same patriarchal ideology that dictates women’s subordination as second-class citizens."
I don't agree with this. I don't think that male transgender people experience transphobia because they are seen as women, I think it's because they are men who are 'presenting' in a way that is not usually associated with men. I would say that it's more like homophobia than misogyny.
"Second, transphobia disproportionately harms women."
I don't agree that this is true either, given that most transgender people are male, not female. In fact, females who identify as transgender are barely mentioned anywhere. However, the primogeniture exemption could prove that TIMs have more rights than TIFs so I suppose I can see the argument that transphobia does disproportionately harm women.
"The history of feminism tells us that women know all too well what it’s like to not be seen, heard or treated with fairness and respect."
I don't see how this means that transphobia is a feminist issue, just because men treat women like shit too. Men can treat gay men like shit too, but that doesn't mean that the treatment of gay men by other men is a feminist issue.
Transphobia aimed at females who identify as transgender could be classed as a feminist issue, as they are females, but somehow I'm not sure that's what the article means.
All that article proves is that transphobia is an issue. But it doesn't prove that it's a feminist issue.