...would it not be accurate to say that "groups" or "objects'" reside on layers????
Still highly inaccurate, for the reasons elaborated.
I watched you a few days ago make exactly the same mistake as your pseudo-experts, either because you're as unfamiliar with the tools as they or because they've simply spoon-fed you your belief.
Zebest starts with the presumption that a forged document was created in Photoshop. Not inappropriate, since that's the product she writes books about. Photoshop has a Layers panel. In Photoshop, every entry in the Layers panel is a layer, principally because a Photoshop image is a flat list of layers, and each layer is conceptually the same thing.
Illustrator also has a Layers panel, but it behaves very differently than Photoshop's. When she imported the Obama PDF into Illustrator -- a tool wholly unsuited to analyzing PDFs -- Zebest noted several entries in the Layers panel and wrongly interpreted that to mean that each item was a layer, or somehow in its own layer. In fact Illustrator reported exactly one Layer -- the default layer
Illustrator creates to import content into. The other entries in the Layers panel were the contained objects.
PDF objects are not layers. They're not "commonly called" layers. They aren't organized into layers. They aren't treated as layers. They have no properties analogous to layers in the Adobe creative products. Zebest committed a simple novice error and has been trying to tap-dance around it ever since by silly word games.
Every scanned PDF that undergoes optimization produces a PDF containing several objects. Optimization works by replacing elements of the scanned image with simpler objects conveying the same intent with fewer bytes. I've posted instructions for how you can demonstrate this to yourself.