Screw FSF approval.
Purism is about making a machine that is owned by their owner. This is a much weaker condition than what the FSF wants. E.g. a software with a license that allows reading it but not passing on modified is anathema for the FSF, but for Purism’s goals, it would be fully sufficient.
Of course an FSF-“free” distro would be better, but rolling your own distro is going to be very expensive over time. Collaboration with an existing distro, or just acquiring the critical mass to attract enough people that it gets maintained by a community, those would be smarter moves IMHO.
BTW “pushing upstream” is really easy, at least for drivers. Just submit your patches to the Linux kernel people, distros will soon pick them up.
While waiting for the patch to make the roundtrip Purism -> kernel -> distro -> PureOS, the driver can be written as a loadable kernel module as an interim solution.