<20g is just the threshold that's guaranteed to put most people into ketosis, but the exact number will vary by person. <50g is also a common threshold that works for many people. I'm not an expert on what qualifies as a "ketogenic diet" but personally I would say that if your diet consistently leaves you in a state of ketosis, then it's ketogenic. I would hope that any study on ketogenic eating is actually measuring blood ketones to determine this.