Why don't some papers with proposed prototype have any user study ? How did some papers with proposed prototype can make it through publication without conducting user study ?

This paper for example 
Reflex HMD to compensate lag and correction of derivative deformation, it came up with experimental stage, evaluation, and conclusion based on their experiment result. How did they even get the evaluation data and came up with the conclusion without conducting user study ? 

Even if they did evaluation by themselves, wouldn't the result be doubtful then ? Just how important is it to include user study in your publication ?
Producing a prototype that solves an interesting problem constitutes a research contribution. Hence, it isn't necessary to conduct a user study. This still holds true when several solutions are known. As more solutions emerge, the original problem becomes less interesting and merely producing another prototype is insufficient to make progress. Hence, it becomes necessary to distinguish new prototypes from previous prototypes. User studies are one way to distinguish new prototypes.