Energy Is Not Conserved

We already knew that energy isn't conserved on cosmological scales. Energy conservation comes from time symmetry, and an expanding universe isn't time symmetric. That doesn't even require dark energy; it's just plain general relativity (even if the cosmological constant is zero).

That's not what the paper is about. The paper is proposing that maybe you could detect dark energy by trying to cancel out all of the other sources of energy, by using two interferometers. It's clever.

That could potentially manifest as a violation of the law of conservation of energy, but that's not really the important part here. We already knew that part.