Pro tip: You can pick any note you want as the starting pitch, and do realtive pitch from there. It doesn't matter. Pitch is an affine space, there's no 0 pitch!
Oh of course, if you just want to practice sight reading on your own, correct pitch is irrelevant, but if you want to use an app like this and have it follow along and check if you’re accurate with your sight reading, you have to agree with the app on pitch!
In music school, I was in the position of having excellent relative pitch and zero perfect pitch. In the ear training courses, I actually scored better than the peers with perfect pitch, as it seemed they had difficulty with a lot of the exercises, particularly the by-ear transcription of the four-part bach chorale snippets.
Pro tip: You can pick any note you want as the starting pitch, and do realtive pitch from there. It doesn't matter. Pitch is an affine space, there's no 0 pitch!