You're making points that have nothing to do with @v0lume4 quote nor the topic of the article.
This is about profits and percentages of said profits. The resale value of used iPhones and Galaxies have nothing to do with those profits. In fact, each sale of one of those "used iPhone keeps an unbelievable value" actually takes profit away from Apple.
Apple never discounts phones at mid life? Uh, that's not true. Outside of the debut period, iPhones can be found discounted as low as BOGO (buy one get one) pricing through multiple carriers. I think AT&T is still running that promo now. Speaking of AT&T, they're one of Apple's biggest customers. Not you and me. The vast majority of Apple's iPhone profit comes from their primary customers: telcos, big box retailers, etc. For the most part, Apple doesn't get paid when you go into a store to buy a phone. Apple was paid long ago when that phone was put on a boat, truck, or plane and shipped to their customers.
As you say... AT&T pays their standard price long before they run their deals.
They don't offer BOGO iPhones because Apple gave them that deal! They accept the loss of the profit from that phone in order to lock in two new customers for 2 1/2 years (these BOGO deals are done via bill credits over 30 months; if you leave during that time- you must pay the remainder left on the phone).
OP was indeed correct- Apple does NOT discount their phones midway in the year before the new one drops.