I have got the exact opposite lesson: a 1 year old iPhone has lost 25% of its value, and it loses another 25% of the original value the next year. So you lose the same amount with an older phone
In other words - imagine a rich person buying a new iPhone every year and selling the old one to his poor friend. Both of them eventually pay the same amount for the phone, but the poor friend got a used one.
I think you might be misreading the graphs. According to the article a 1 year old iPhone depreciates by about 40%, and a 2 year old iPhone depreciates by around 65-70%.
Thus, a person selling their 1 year old iPhone would fail to recoup 40% of the sticker price overall while someone buying 1 year old phones and selling them a year later would fail to recoup all but 25% of the sticker price, give or take.
In other words - imagine a rich person buying a new iPhone every year and selling the old one to his poor friend. Both of them eventually pay the same amount for the phone, but the poor friend got a used one.