Earlier quoted context omitted.
Pinephone has multiple compatible os-s, while most android phones don't have alternatives and become unusable if android drops support for them. Also, Google and the big OEM-s have the incentive to break compatibility to provide you with the newest gadget, which is not true for the pines. yet.
Plenty of android phones have alternatives. It's what custom roms in the title of this article is referring to.
There's no way to get the latest AOSP on these devices forever. Hard work is required to port any new version and that's not always possible because AOSP comes to rely on newer kernel versions but you are stuck with the heavily customized one (+ the blobs) the manufacturer provided.