After Pine64 went to all the trouble to make a series of branded early developer editions for various distros, and ongoing community goodwill outreaches, I didn't expect something like this to happen.
IMHO, any handheld hardware right now wanting to be open and sustainable Linux should probably start by making sure it's solid with PostmarketOS configured with a mainline kernel, as a good test. (PostmarketOS is a litmus test for purity, and has had a bunch of work to make bringing it up easier than the usual server/desktop purity test of Debian.) Working Phosh and Plasma userlands running atop PostmarketOS are good further tests. Supporting additional distros, userlands, and hacking is also important.
(Even if the hardware is something like the Purism Librem 5, which has invested heavily in lots of good distro work, if PostmarketOS with mainline kernel boots cleanly and runs solidly, that's a good sign.)
While Debian has more strict policies on software licensing etc the musl component of postmarketOS pretty much makes sure that no closed source software will ever run in userspace. The main difference is that postmarketOS does deal with firmware for hardware while Debian doesn't. Hopefully we someday get at a point where even the firmware for these devices isn't problematic anymore.