That app does have some search function where if you input your case number and some derivative of it by increasing/decreasing the last few digits then it pulls up upto 500 case numbers before or after your case that can be found to show results in the website. It ends up showing a list of all cases categorized by filing type (I-485, I-765 etc), date, status and last change in case. It does sometimes shows adds and popups, hence I was being cautious.
That random app does pull up all existing cases. It does feel odd but as the website requires only case number hence an api can be used to run a string to get all available results.