Inflation in the early universe, there's still quantum fluctuations. This gives rise to density perturbations we see in the cosmic microwave background. It doesn't just lead to densities; it also leads to fluctuations in the gravitational field as well. Those have a unique signature that is called the B mode. The E mode is predicted to be there whether or not there are gravitational waves and it has already been observed. There are other things that can give rise to B modes in later universe that don't give us evidence for inflation or against it.