Watching how surprisingly well this works for Purism and System76 for laptops, PCs and servers, plus others for niche products (e.g. Turris), it is understandable to view a smartphone commercially feasible this way too (with an "own" operation system), at first sight.

But I have the impression that they massively underestimated the task of creating and maintaining a completely new GNU/Linux based software stack for a smartphone.
Hopefully Purism will survive this endeavour.

OTOH, just to provide the hardware and FLOSS boot-firmware (usually Coreboot with a specific "payload" for boot device discovery and loading an OS kernel), as Raptor Computing Systems and PC engines do, is currently not well feasible for a smartphone: A common boot-loader mechanism is usually missing.

... and when!