The input points of  Triplanar node (color_texture, float_texture and height_texture) are File-Image data type and does not require any node connection to them, so they have to be ignored from this aspect. It is shown on the VOP Network because it is an input parameter which accepts the path of any texture, but that does not mean that you can connect another node to it. Also you can refer to the color that the input points have. The color of these points is different from the color of other points (like Color data type) which accepts node connection