It's patently ridiculous. If it's supposed to refer to religion (and, I presume, Protestantism) why have I never heard anyone refer to a hun church or a hun minister or even refer to the kirk or the General Assembly or the Synod or etc etc etc? The "but it means Protestants when it's written on a wall in Northern Ireland" argument is one of the silliest I've ever heard. Basically, Rangers fans don't like being called it and the only way to try to stamp it out is by yelling "sectarian". I presume you only want to be referred to by self-given epithets? If that's the case I'm sure you'll reciprocate with everyone else
I've personally don't know any Rangers fan that resents being called a hun. It a generic term for a Rangers fan, although I have seen it being used in a context to refer to proddies.
I know you don't know them personally but it seems Chapper and Medro do. Or at least, in Chapper's case, thinks it's on a par with a word like taig. Medro, I think, is just trying to be offended and knows deep down that Hun doesn't mean Protestant or Anglo Saxon (whatever Anglo Saxon means) The two huns sitting next to me in work just now both think it's sectarian. A quick swatch on Facebook or Twitter would show that there are loads of people thinking like this. It's the result of a proper misinformation propaganda movement. With the arch-****-in-chief who wrote the blog below leading it. http://chrisgraham76.wordpress.com/2011/12/30/hun-when-does-it-stop/
I have seen it used in a context where it was clear it was being used to refer to a proddy. Is that sectarian? Dunno. Maybe to some.