I know this probably doesn’t add much to this topic, or help much, but I’m thinking they somehow edited the normals, or the direction each face/vertex “faces towards”, making Roblox believe all of them are facing straight upward, lighting all of them regardless of where they’re located on the model.
I don’t know how to do it, but messing with Blender’s NormalEdit modifier gave me this result; There’s still shading on the side facing away from the camera, but it’s closer to the unshaded look of those models.
The only time I’ve gotten that exact look is when I’d sometimes upload a 3D model to Sketchfab and it would incorrectly have all of the vertex normals facing upwards. I wish I knew how to do that on purpose within Blender so I could help more.