Didn't work for me either. I couldn't budge the back using any technique with my palms.
I used RegShoe's method (but without the watch tool) and got the back off. I used a dremel tool with a flat-tipped burr bit to grind two small divots on the back surface of the Pixel, being careful not to go through the plastic. I then used a pair of fine needle nose pliers, spreading the tips and putting one into each divot. I could then rotate the pliers in those divots to get the back to come off.