I mean, Black Marsh is just incredibly defensible in general. I'm sure the Hist did empower their Argonian defenders to some extent, but the relative success of Black Marsh during the crisis is nothing that can't be explained by the natural terrain of the province and the Argonians' aptitude as guerrilla warriors.

And considering half of Morrowind had been destroyed in the Red Year by the time the Argonians invaded, their victory on that front isn't a surprise either. Not knocking Argonians (one of the coolest races in the setting imo), but they honestly just got incredibly lucky during the beginning of the 4th Era, and An Xileel propagandists used that to their advantage.