SETI isn't a theory. It is an institute pursuing a hypothesis through experimentation and observation. The hypothesis includes the ideas or assumptions that if extraterrestrial life exists and if has a technology to broadcast signals then SETI should be able to eventually detect certain narrow band signals with certain characteristics.
At least we agree that it isn't a theory, but I also wouldn't call that a hypothesis, more like a speculation. If it were a hypothesis, you'd be able to at see, at least vaguely, some way toward falsification.