it was Purism’s choice to start their own Debian-based PureOS instead of just helping develop PostmarketOS or another existing OS for less money

PureOS already existed for Purism’s laptops. Purism didn’t start PureOS for the phone.

was Purism’s choice to start Phosh instead of just helping develop GNOME Shell

It was the GNOME Shell folks themselves who said not to use GNOME Shell on mobile because “it’s a trap”.

which Phosh appears to be heavily based on

phosh was written from scratch.