His position (I've read his book) is that if we find extrasolar objects that are easily produced artificially but require incredibly exotic conditions to be produced naturally, materials we have never seen or imagined yet, then the mainstream view is to assume the natural origin. But he thinks it's reasonable to assume that we are not the first technological civilization and as far as we know, the galaxy could be full of drifting alien space junk, and from that point of view it's reasonable to first think of sheet metal rather than spears of frozen hydrogen for example.
"Any sufficiently advanced technology is indistinguishable from lumps of metal and random chunks of rock" At least when it rains down on us as meteorites.
