Okay look, it's a zip. Full stop. No arguing. That "SHA-1 layer" and "metadata" are the
zip header. He can use all 29 bits in the header because he's not compressing the contents of the archive. He can tell you what he wants, but file formats don't lie. It's a repurposed and slightly rebranded zip is entirely synonymous.
That's still not what he did, as I explained a few pages back when I directly replied to you. To reiterate, it's to verify file integrity and to prevent bricks from flashing incorrect firmware.