“But if this is alien technology, how does it explain Oumuamua’s silence? SETI (Search for Extraterrestrial Intelligence) scanned the object while it was still near to Earth and found no signs of radio signals.”

Why do you think this “silence” argues against Oumuamua being alien technology?

Radio communication has been used by us humans for about a hundred years, a very small fraction of the history of our species. It’s certainly conceivable that other intelligent beings might also use radio. But can we assume that any and every spacefaring civilisation does so?