Do many people really have a stockpile of working old phones?
From my observations, phones get destroyed, used until the battery swells and breaks them, or handed down to kids or less careful users. No one I know has a bunch of old phones that are still useful but unused.
There are recycling and trade-in programs that could collect compatible phones and pass them on in bulk.
I have Nexus 5, Xiaomi A1, Redmi Note 7, Samsung S7, and a Kindle Fire HDX, all running either LineageOS (Kindle) or PostmarketOS (the rest). PmOS ones run some not very demanding containers (scrapers) on k3s.
They're not useful as phones, because the battery, screen, radio, etc. are damaged; but they may still have a working CPU inside, which would be sufficient for this project.
I have a drawer full of old phones with broken screens, obsolete chipsets, etc. I usually buy phones rather than get them through a contact plan with trade-ins though.
I did before the switch to volte.
I recycled them wheb carriers decided to block a bunch of phones.
I have a large amount, some are occasionally useful when debugging.
The article mentions the Pixel Fold. I suspect Google possesses every Pixel Fold ever made.