This is kind of like picking a scalar input function with a constant derivative to assert that newton's method is just error feedback. -- in that special case t

This is kind of like picking a scalar input function with a constant derivative to assert that newton's method is just error feedback. -- in that special case that's all it is, and you won't get an intuition that newton's method constructs and solves a local linear approximation from the function's Taylor expansion unless you look at a more complicated case. Same applies here, single input with a trivial model, kalman is just a lowpass filter.