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He literally gave an example where that did happen.
Two answers, shamelessly copied from reddit.
1. Actually, the cold welding of gold is fairly common on earth. It's the basic mechanism for the formation of gold nuggets in waterways. On a larger scale, very flat contact surfaces and significant pressure is required to produce welds of noticable strength - as is the case for other metals.
2. It does happen, with sufficient rubbing or if the gold is sputter-cleaned in a vacuum. Oxide layers aside, all materials exposed to the atmosphere have at least a monolayer of adsorbed "grime" on them - carbon, hydrocarbons, sulfur compounds, etc. This layer, along with surface reconstruction to minimize dangling bonds, is why gold objects don't spontaneously cold-weld to each other.