If you want a non-Intel desktop, you could also look to any of the high-performance AArch64 or Power boards with PCIe slots in them, and slot Radeon in.
The chip they're using is from the i.MX line with Vivante GPU. The reason this chip has open source drivers is because of the etnaviv project, which was funded by bunnie and the backers for his Novena project. So in a sense, the laptop you want already exists and in fact came first. It's just a little pricey and kind of rough around the edges.
Are there actually open ARM CPUs? My very vague understanding was that ARM claims that their ISA cannot be implemented without infringing some patents on some instruction encoding tricks they hold.
So yes, I guess the hardware isn't open, but all the software/firmware on can be (for example the RK3288 [3]) (of course with the exception of the GPU).
1. https://libreboot.org/faq.html#intelme
I'm very surprised that the phone will have an open GPU. I've been looking for a laptop replacement for ages that has an open CPU and GPU. While I can avoid Intel like the plague and go straight for an ARM CPU, the GPU always tends to be a closed source binary blog.
If Purism can crack this and get a full open source phone, I hope they build a lightweight laptop using the same sort of specs. This could mean avoiding the whole ME cleaner requirement for their Librem laptop range, removing the spyware [1] from all Intel chips.
1. https://libreboot.org/faq.html#intel