Finding B-modes in the CMB (which aren't due to foreground contamination) would be evidence for gravitational waves, because they cannot be produced by density perturbations (to first order, is my understanding). My question is how can we jump from "We have found gravitational waves in the CMB" to "this is evidence of the quantum nature of gravity"? (this is a common enough statement in science publications on the topic) According to this answer, the size of the universe before inflation was ~500k Planck Lengths, and after inflation it was the size of a grain of sand. So during the entire process, the scales involved were many times the scale we usually associate with quantum gravity. It seems to me that finding gravitational waves in the CMB would give more evidence for the validity of classical GR, not of quantum gravity.
Just to clarify, this question is not "do the B modes found by Planck suggest an observation of gravitational waves" (since that is under debate) or "does the gravitational field have a quantum description?". I am just interested in how one connects the scales associated with inflation to the certainty that observation of gravitational waves would probe the quantum nature of gravity.