I must admit that I don’t frequent Slowtwich that often, and stuff like this is probably why. While I have zero first hand experience with the R9100P, it’s very clear (as in zero ambiguity) that this power meter doesn’t work. While some of this is inherent in the asymmetric design of the 9100 crank, most 9100 based power meters (e.g. Stages, 4iiii, Pioneer), while imperfect, at least function in a predictable way.
This magnet is an anachronism to some. The PM is top cabin in a number of ways (battery life, it’s integration into an industry-leading crank). The magnet on the frame is to log cadence. Wouldn’t Shimano solve this with an accelerometer in the crankarm? Shimano views the magnet as a feature. It thinks adding an accelerometer is an unneeded complication. Fair to say others feel differently.