The assumption here is that no force has acted on the energy arriving enough to distort two arrays on earth.
No physicists here but you don't seem to be either. Irc from what had been said with the first event gravitational waves pass through matter and aren't disturbed like em waves, that's why they supposedly open new doors. Secondly, just take it at face value, if they say so it is so, it might seem improbable and "horribly prone to manipulation" but they probably thought of that and have machines precise enough to compensate for whatever effects you or me imagined e.g. "LIGO is designed to detect a change in distance between its mirrors 1/10,000th the width of a proton"[1]. It's like your grandma, never having touched computers, expressing opinions on the next release of some developer tool.
Also they travel at the speed of light, the rotation of the earth is pretty much constant (if not negligible).