But aren't there plenty of used expensive phones from the last 5-7 years that are more or less equivalent replacements for new cheap phones? Apple alone sells 250 million phones a year.

The article takes a while to get there, but it is focusing on a set of companies I hadn't heard of ("Transsion, Oppo, Vivo, and Lava") that buy components from last gen smartphones to make cheap devices to sell in the African and South(/east) Asian markets.

Presumably the supply of 5+ year old used phones that fully work is not enough to meet that demand, which is why these frankenstein Android companies exist.