Fatphobia is hardly an insider term. Its meaning is self evident unless you consider, say, homophobia to be an equally arcane term. All forms of irrational hatred spring from fear, and fear of becoming or being perceived of as fat with all the negative steretypes that go along with it is at an all time high. Despite this, however, fatphobia, (or sizeism if you find the term less oblique), is a form of prejudice that is scarcely recognised let alone challenged by those who aren’t on the receiving end of it. That’s why the vast majority of people writing about it tend to fall inside the stigmatised demographic. That’s pretty much how most social justice movements start off. Once a significant number of marginalised individuals decide they’ve had a gutful, that’s when the liberals start listening and the bigots start whingeing on about Political Correctness Gone Mad®. Eventually everybody else cottons on and, with luck, some progress is made.
As for not liking the tone of articles written by angry marginalised people, have you considered you aren’t necessarily the target audience? I can’t speak for the author but when I write about fat politics, I generally do so in the hope of striking a chord in the fat person with some internalised sizeism, rather than the arsehole who put it there. When I do engage in debate with a fat-basher I do so with the primary intention of giving them enough rope to verbally hang themselves and they rarely disappoint. That way they do my job for me. Finally, it’s been my experience that when people fixate on a tone of voice they don’t care for, it’s generally because that voice is saying something they find uncomfortable to hear.