I think those memory restrictions are meant to be used with the 8GB dev kits but for the 'new' 8GB retail PS4.
It really makes sense that they needed 3.5GB for OS/debugging for a 8GB dev kits targeted at an 4BG PS4.
That would explain why they would 'give like candy' those oudated 8GB dev kits.
The proof that those documents are destined to 8GB dev kits, from eurogamer:
but the docs are clear that the extra RAM here is only available for application development, presumably in order to house debugging data