While the majority of mods where indeed not made by him, he basically streamlined them and merged them into on "super" mod that added a large variety of features that total war fans had been asking such as editable turn duration, tech research rate, armies of over 20 units, more balanced factions, realistic effects, option to turn off tracers, making similar units such as "line" infantry of empire actually different units, more playable factions, turning forts on/off.
Most of which CA denied where possible in the specific games they where modded into and to this day some of which they still claim are impossible on the engine (aka turn real time duration options or tracer toggle etc). Empire total war became playable thanks to his mods long before CA released patches and to this day still fixes issues CA where unable to do themselves.
Most notably the AI. While its far from perfect if you compare the AI in darthmod to vanilla empire/napoleon/shogun2 you notice it actually takes the initiative in a lot better manner and actually attacks weak spots in your line.