I looked (briefly) online for evidence of the factory claim and found none.

GP didn't give enough context ("MY shoes are weird so I extrapolated" vs "I know this is true; here's proof."), but the casual one-off scenario is absolutely a showroom level of "random happens".

Babysitting all those individual shoes and styles and sizes seems ... expensive, logistically.

In fact, if this were a thing, I'd imagine they'd start offloading them as though it were a cool feature. "Want to mix-n-match shoes? buy just a left!"