Correct. Threre is a TRUE North pole (stationary spot on the earth - think funny pole stuck in the ground). There is a MAGNETIC North pole that moves around slowly over the years. Planes fly MAG headings, because the magnetism does have an effect on instruments depending on where you are on the earth. Charts, Runway headings, Navigational aids all have small changes over time, which means NGA and private companies like Jeppesen are constantly updating data on a monthly basis to make sure the devises used for precise navigation (not to include GPS) are accurate based on the magnetic deviation for that area of the earth.
I will also throw in that some military aircraft have Internal Navigation Systems (INS') that you align prior to takeoff. These systems use ring laser gyros that are so sensitive they can sense the movement of the earth. To align them, you have to precise coordinates of where your plane is on the earth, what direction the plane is facing, and then you need to turn the plane 90 degrees off heading and let the system sense to rotation to complete the alignment. Once the INS is aligned, this system is a completely self-contained system that can't be influenced by enemy actions. It's a good GPS backup should GPS jamming occur…
I hope I answered your question…Magnetic influence I believe could affect instruments on both a sphere earth or a flat earth, so my brain might not be following how that factors in to proving one way or the other.
Again, my hands on understanding of the INS backs sphere earth…just another real life influence that supports…IMO