Is a recording of a chord merely a superposition of recordings of its constituent notes?
It's close enough for most practical synthesis purposes.  However in real polyphonic instruments, there tends to be some coupling between the various oscillators that can, for example, synchronize notes that are close enough to whole number ratios.

For other ensemble instruments, the coupling can work via the hearing of the instrumentalists and minute corrections (of embouchure, fingering pitch, and so on).

And for nonlinear effects (like accordion bass mechanic sympathetic vibration or a distortion pedal for a guitar or a distorted amp), the results are definitely non-linear.

But more often than not, just adding the samples is a good enough approximation.