I think the “not cisgender” definition came about in the last ~10 years or so, in community discussions irl and on social media (I would not be surprised if it started and spread on twitter.) I might be misremembering but I think it happened right after and as part of the same process as the community rejecting transsexual vs transgender and medical gatekeeping in general. I remember a lot of non-traditional binary trans people and all kinds of nonbinary trans people challenging the fairly strict “rules” of gender conformity+ levels of medical transition someone had to embody to be considered legitimately trans. The umbrella “not cis” concept came about after a critical mass of nonbinary people, and gnc binary trans people pushing back against “if you haven’t done xyz you’re faking it and aren’t really trans,” rhetoric.