Yup. There's probably a lot of little such things where the model deviates from reality. Might be a way to exploit the friction in the various hinges, etc. It may be easier to turn on a low-friction surface where the ground friction doesn't overwhelm these effects, etc. Fun to play with, but the end result is still that even if the real world doesn't give us a "truly" unsteerable bike, I am still surprised that this curve between "steerable bike" and "steerable trike" passes near "zero steerability" at all. I would not have guessed that.