This discussion is being position as black and white. Any control points in an adventure is railroad. Only an adventure without any constraints is a sandbox. This is a false choice that doesn’t take account of how adventures have adopted the Sandbox format and improved adventure design with it. While still maintaining a fully fleshed out practical campaign product.
No, what makes it "not a sandbox" is when noodling about it in it and filling in blank spaces isn't particularly meaningful, because the optimal choice is always to hit objectives quickly and move on.