How can hardware be open without build documentation? Unlike software which is infinitely replicable and open unless obfuscate, hardware is private by default as the method of construction is effectively the "source code" and can not generally be derived without direct access to the hardware in question and disassembly. Dissassembly without reassembly instructions can only derive vocational information, and reverse engineering is required to translate that to assembly instructions which themselves are likely to differ from the prior engineer's method.

Hardware is only open source if its assembly documentation is made openly available.

"Open source hardware is hardware whose design is made publicly available." is the first sentence of the link you provided. Viable Hardware Design includes assembly, because as any hardware engineers will tell you a schematic is only valuable if it has been proven possible. Schematics themselves are documentation, design of hardware is documenting hardware otherwise you are crafting not designing.

End rant, lol.