hmm, really not sure about the 'has it been a photo once?' thing..
but the fact that several pixels look very randomly put and that the aa of the bushes and walls look to smooth/random to be handmade let me tend 70:30 to 'there was a photo involved.. somehow'
I'll have to go with Lawrence here, too. I think that he wouldn't waste a reputation like his by just color reducing. There's nothing in the actual pixels that show that this is anything but hand-pixeled. The dithering isn't random, lines are straight where they need to be straight.
That being said, the composition/lighting of this piece is beautiful. I think Lawrence did a great job capturing the feeling of dusk. I'm surprised that he says he didn't use ANY reference pics, but if there's no evidence that it wasn't 100% hand pixeled, and if Lawrence says that it's 100% hand pixeled, then why can't it be 100% hand pixeled?