I'm your height and use to weigh 120lbs.
I bulked up to about 153lbs..
I'm down to 140lbs now because I had pneumonia. But my arms are a little bigger than they were when I was 153lbs.
I'd say they are mostly right when they say there are no hard gainers. We're just bad eaters - inconsistent eaters. We have some big meals here and there. But we are like gas guzzling, high performance cars - we burn through fuel like a V12 engine. We gotta keep a steady, uninterruptible supply of gasoline, because our engine is always revved high, it's never purring idly.
I've done crazy-ass workouts. Like full body workouts for two to three hours, every day, four or five days a week. To workouts every other day, to ones that were only once a week. High reps/lower weight, low reps/heavy weights. I've even done two workouts per day.
It'd didn't really matter what type of workout I was doing, or if I was over training: as long as I was eating enough of the stuff I needed to eat, I got bigger. It was only when I wasn't being VERY consistent in eating CONSTANTLY, that I wasn't making gains.
I always thought I use to eat a lot. A lot of us hard gainers have that mistaken impression. It's not till you actually put on some weight that you look back and see how little we actually did eat.