And yet, vacuum-welding is a long known actual thing, and even without vacuum right in ordinary human environment, by good, non-trivial, but not magical surface prep. But it's probably more than just proximity. In vacuum there is probably also evaporation and a form of electrolosis.

No that huge a void.

Optically flat polished metals, such as guage blocks, will form a few metallic bonds when pressed together.

Of course, but the OP asked what distinguishes one group of, say, Al metals in one work piece from another --> The void and xtal misalignment.

Optically flat isn't even needed per se. You can weld metals by slipping two blocks a bit under pressure side by side just like that in standard atmosphere, and when there's some sort of low-pressure environment it just takes that much less of a slip. No need to go down to space-grade vacuum at all.

Surprisingly, explosive welding is considered to be a cold welding process. Explosives are used to overcome the limitations you describe.