Literally what I just said
Your wording can easily be read as them shipping 3d models and rendering them as 2d at runtime. It's a weird way to say the game is made up of 2d assets.
I wish that were the case, then they'd have released the (original) 3D models!
The post had me wondering if factorio had done like [1]Prodeus did where it's all 3D models, but by default is viewed as 2D sprite animations.
[1]https://www.prodeusgame.com/website/index.php