Cannot speak for that app at all, but on the principle of how higher end equipment measures it, it seems within the scope of what a phone or similar device could process, seeing as it seems to mainly use digital filtering and process based on a ADC data feed, with unless I am mistaken no significant hardware in the path, however where I would take caution is that I am not sure with modern phones you can ever access the true "RAW Feed", for the internal microphone, I would assume its run though atleast some level of filtering before its handed to the application which may alter the readings your expecting
At a minimum you could likely also approach this with a PC's mic in, and run an android emulator to allow you some better hardware in the mix,
Best reference I was able to dig up for how the measurements are done was below.
http://www.miko.hk/IFR/885.pdf