Asking about it in the official subreddit resulted in responses ranging from unhelpful to hostile.
After looking at this repo more closely, this really is pretty suboptimal. build.sh is incredibly flexible, but yeah- no documentation on what you'd need to build a non-pfsense branded product.
That's pretty weird, given how much of this system seems to be geared towards being generically applicable and has some replacements aimed directly at rebranding some literal occurrences of "pfSense".
I really do enjoy the platform, I have dozens of Netgate SG-series devices deployed to clients and they work great, but I do find it hugely disappointing that it seems very clear that Netgate does not want anyone else producing builds of pfSense-derived software even if it's properly rebranded.
Their official claims that it's purely about the branding falls apart when you compare what's involved in building "nonSense" compared to Chromium, Iceweasel, etc.