is "spaghetti sort" really O(n) (even as a thought experiment) ?
The folklore analysis of spaghetti sort assumes a model where one can extract the longest noodle in constant time, perhaps by lowering a horizontal plate ("hand") over the vertical noodles until it stops, and then extracting the noddle touching the plate.  (Apparently the plate is pastamagnetic.)  Within this perfectly well-defined theoretical model, the analysis is clean and obviously correct.

So is the pastamagnetic-plate model reasonable?  For up to a few hundred strands of spaghetti, sure.  Beyond that, obviously not.  So should we make the model more reasonable by taking the physical costs of moving the plate, sensing collisions, or  choosing the right noodle?  What about the (nonzero!) physical cost of keeping all the noddles vertical (or at least parallel), and keeping the sensing plate and the table horizontal (or at least parallel to each other but not the noodles)?  Should we worry about the measuring process causing micro-fractures that change the exact length of the noddles, or the implication from quantum mechanics that there is no such thing as "exact length"?  Holy crap, is the spaghetti-sorting problem even well-defined?!

No, this way lies madness.  The original theoretical analysis is predictive within the context for which the algorithm was designed — manually sorting a handful of pasta #8.  That the analysis does not generalize to significantly larger values of $n$ or other types of pasta does not make the analysis "incorrect".  It merely limits its scope.