It's a noble goal, but they're using Intel hardware. They also don't claim to be free software down to the firmware.
But assuming that's their goal:
They say they talked with Intel about opening up things, but they're far from the first one to try (and not the most powerful ones either, be it by $$$ or influence, unless there's some background story they keep to themselves).
My guess is that they intend well, talked to some sales folks, those promised the moon (and more, if necessary), and when things get serious, the sales persons will be nowhere to be found, while purism is too far down the road to start from scratch.
In the end, purism has to compromise while Intel can celebrate another design win.
So the end result will be that they use the smaller ME firmware (1.5MB is still a lot of unexplained cruft), the FSP binary (www.intel.com/fsp) and coreboot to glue things together - just like every other coreboot-using Intel based design does right now (there are a couple).
The main difference to everybody else will be that purism builds neither embedded electronics that do things no consumer has ever heard of, nor chromebooks.
That in itself is already a nice thing from a coreboot perspective, in that it provides exposure to the project and the issue, but I fear some purism backers will feel let down by such an outcome.
http://puri.sm/posts/purism-software-freedom-deconstructed/ looks like they try to indicate that "this BIOS code isn't all that much". No, it isn't - but it's exactly the part of the code that sets the rules under which the entire system later operates. In this, it's more important than the plain old (say) gimp binary in that huge top bucket, which has not much of an effect on the security or autonomy of the system or user.
I'd love to be wrong here, since either way I 'win' (actually the more I'm wrong, the more I win :) ). I just think it's unlikely given that I'm involved in that mess for the better part of a decade by now, while the purism folks seem to be newcomers with a lot of idealism but few experience with dealing with Intel and their unusual definition of software freedom.