Well yeah, but what is the non trivial statement here? Linear model is linear, and if you add to your model non-linear steps (such as pre- or post- processing)

Well yeah, but what is the non trivial statement here? Linear model is linear, and if you add to your model non-linear steps (such as pre- or post- processing) then it is not longer linear and can handle non-linear data. In the same spirit, you could say that GPT4 is a linear model (token logits) with a very large preprocessing (transformer). Maybe what OP meant is that particular non linearities (splines) allow to keep some goodies from the linear model in non-linear settings. Still it's not clear for me in which settings exactly such models are good enough