Well what parts of Arrival said this?
Project base. Before the shooting starts.
"The Reapers will arrive in this system regardless. If you want to keep them at bay, the Alpha relay must be destroyed."
Paraphrased, of course.
Yeah that was real vague...
They never said whether they were in Transit through the Relay, I guess it's implied or whether they were approaching the Relay on their side and preparing to use it....
I'll play agains...