I think that statement can only be said with the right context.

Building an engine from scratch? 3d is definitely more complicated.

Using an existing engine? Tbh shoehorning 2d sprite based games into engines like unreal or unity is often harder than 3d because it gets less development effort on the engine side put into it.

Asset creation? Depends - the great thing about 3d assets is you only need to model each thing once, whereas with 2d unless you're doing skeletal animation you have to draw every frame, facing direction etc.

But on the flipside creating one 3d model could take just as much time as a pixel art walk animation in 4 directions.

There's also art styles for both 2d and 3d that can significantly cut down on effort (low poly, drawing everything from absolute top down perspective, using ascii characters roguelike style, using simple 3d primitives only, first person perspective etc etc etc).