I believe his point was that:
A. Creating easy to use tools would be a high cost investment that Larian didn't have the money to build.
B. Creating assets as a modder would be a huge time/cost investment.
However, if you are just reusing existing assets. Maps, buildings, characters, etc. you can probably mod an adventure into what exists.
A fun little self contained mystery in the city might be fun. Or a battle Royal in the underdark with players dropped on opposite ends of the map.
