What we don't know is how representative we are. Nor do we know the limits of exponential growth, for they may lie not at the planetary but the solar system level, assuming they’re not fully surmountable in the first place (by some future civilization if no one has done it in the past). A success at finding some kind of artifact here in our own system would at least tell us that an interstellar crossing is not out of the question, but how much further do we want to take these conclusions? The authors raise the question themselves, and point out that " . . . we cannot rule out the possibility that ETI civilization may follow a development pattern sufficiently different that we wouldn't recognize it even if we detected its signal."
[Follow me on Twitter.]