From my understanding, it wasn't that Alpha's couldn't mark Omegas, just that it didn't always take (?)... and Keith could still emit pheromones, he just wanted to be with his omega (and he could still smell pheromones) - there was something else to it (I don't think it was just that he had no memory of who he slept with or who had marked him), but I don't remember. I also think that Josh simply never had a strong scent (unlike Yeonwoo).
In KML both can Mark each other but is different on both, when an alfa is marked he can't smell or react to other omegas and can only be with that person. But when an omega is marked he can be with other people and replace the mark with a new one, the only thing is that ones marked he can smell other altas pherormones but don't react to them the same way as with the alfa that marked him.
In Josh case, he didn’t smell that much to begin with, his pherormones are very low so nobody notice them, then with the mark he has more tolerance to others pherormones, he only reacts to Chase. Also Chase has more selfcontrol than those in KML
The thing is, with what Grayson said in KML, an alpha marking an omega is useless because it can be replaced by other alphas, so if it's temporary why did his mark lasted for years. So you're actually correct with Josh not reacting to other alphas but the one who marked him but when Keith bit Yeon Woo it literally doesn't do anything, hence Alphas marking omegas are useless. It just doesn't make sense to me that here the marking lasted for years but it was established before that marked omegas aren't really a thing.
sorry if the reply's wonky, and redundant I feel sluggish and sleepy rn. But you get the gist, right? Alphas marking omegas = temporary and can be replaced by other alphas, but in this story he was marked and lasted for years, contrary to the established marking 'rule'.
Ya, it's weird
Yo, did marking change in this universe? CMIIW, but in Kiss Me Liar, alphas can't mark omegas, I mean, they can but only temporary. So remember when Keith spent his rut with Yeon woo, and as an act of revenge YW marked him and now Keith can't emit pheromones and can only smell his omega's scent. But in this story it was the opposite. Chase marked him and now Josh doesn't have a scent. Is this how it is in the novel too?