Nobody in the Resistance actually saw that Luke was a projection.
Still, it's gonna be a lie. If the truth about what actually happened on Crait ever emerged, the "spark" would deflate and the First Order would have won.
I want to think it's a brilliant allegory for the First Order representing Trump and Luke's sacrifice being the "fake news" that will bring him down, but jokes aside, it's so depressing that Luke's legacy is a lie. It completely sour the movie for me, but I'm the weird one probably here.