The FSK discriminator algorithm written for the book was only tested with frequencies that differed by at least a factor of 2. Tin Can uses a different algorithm that can distinguish frequencies that are much closer, so the 4000/2666 frequency pair does not present a problem for Tin Can. My guess would be that the published discriminator code might have trouble locking on to the bit pattern when it's only a 1.5x difference between the frequencies.

Another possibility: if you are using the built-in microphone to get the sound into the app, then you will have additional noise and distortion in the signal, which this algorithm cannot handle. Tin Can's algorithm was designed for over-the-air sound transmission and it can accept quite a bit of background noise without errors. The published code has only minimal noise rejection, so it requires a wired connection to the mic port to operate. The low-frequency test sounds might work with the microphone, but the high-frequency sounds would not.