You are right, it probably belongs on the Electrical Engineering.
However, I think questions may also belong on Robotics if it was somewhat more intelligent - not necessarily requiring both a mechanical aspect as well as an electrical aspect.
While the question you have referred to may be solvable by the users on this site, the application is not robotics, and I think users on this site would be more interested in questions more directly relevant to robotics. Therefore, we should probably reject other questions of this type.
However, it is not possible to draw a clear line, so in other cases where it is not clear, we should lean on the side of leniency.
Note: Just because a question involves both mechanical and electrical aspects, that does not automatically make it a robotics question. For example, Mechatronics includes areas such as automotive engineering, transportation systems, consumer products, and other areas. However, we would not consider the design of a "plain old car", train/rail system or aircraft to be robotics, unless they were autonomous and/or RC models. Nor would we consider consumer products such as blenders, dishwashers, washing machines to be robotics.