> Fine and true, but setting aside the principle of the matter, did anyone actually prefer Biden over Harris?

Probably not, but does it matter? Biden was also not chosen in anything resembling a democratic way. US political primaries are not democratic.

The general population being presented a choice between two options that were selected by two ultra-partisan entrenched entities is not democracy.

To have a system somewhat resembling democracy you would have to either (1) open primaries to everyone regardless of party registration with no control by partisan organizations over who gets nominated or supported (which would mostly defeat the point of having political parties at all) or (2) have a more proportional system where it is meaningfully possible to create new political parties that gain a nonzero share of representation.