That's a line being pushed by various people for their own reasons. Ukraine is not a NATO proxy. Ukraine is begrudgingly supported, mostly because reflexive anti-Trumpers in the US saw that Trump was unsupportive (so they jumped on the opposite position, not with much enthusiasm) and reflexive neo-cons saw another war to meddle in. Before Ukraine provided yet another angle to attack Trump, it was just a fairly corrupt eastern European country in which most western countries wanted very little involvement.