Interesting app! Glad they don't claim you can now throw away your instruments, but instead have education in mind.
Out of curiosity: Could you please test what range the magnetometer on your phone has?
Reason for the question: As an application for the magnetometer section of the app, they propose to do measurements with magnets. However, the magnetometer (usually 3D) in a smartphone is not designed for that, it is supposed to aid navigation by measuring the earth's magnetic field, which is on the order of 50µT. So one would expect the range of the magnetometer to be not much larger. Since modern rare earth magnets are stronger by 4 or more orders of magnitude, one would expect the magnetometer to be saturated in proximity of such a magnet.
Caution (not knowing which technology is used for the magnetometer in your phone): Some magnetometers, like GMR type, have a maximum tolerable magnetic flux density, perhaps 100mT, above which they are damaged. This limit should be outside the dynamic range, so it appears wise to stop bringing a strong magnet closer once saturation of the output signal has been reached.