You should read the article
There's a linked article from "The Information" that I suspect goes into more detail, but it's paywalled.
A sticker would be noticable. Apple has forced the lens suppliers to add a laser mark with a serial number as they are created, by looking at what serial number screens get through to final production they can see how many get broken/fail to meet tests etc. It doesn't in it self reduce the defect rate, but lets Apple know what the defect rate is, which for some reason the manufacturers are reluctant in telling Apple
My suspicion is that an unknown but nontrivial share of the 'defects' were of the 'fell off a truck' variety rather than a true production problem.
Alternatively, Apple might have contracts that pay its suppliers 'cost plus', such that Apple is on the hook for used supplies without giving the vendors a direct financial incentive to reduce defect rate.