They may use return to SERP rate as a stronger signal. If a user views a page for under X seconds and then returns to search, one may assume that it wasn't relevant to their query. It acts similar to CTR as a ranking factor, but with an extra step.
My only interpretation is that google is using a intentionally narrow definition that is misleading. I could certainly be wrong, but it seems preposterous to me to claim that google never uses information related to users clicking through to search results to inform search.