Use cement board without plywood for outdoor kitchen
For a vertical application movement is a bigger concern that weight carrying ability.  If the cement board is framed well (studs every foot) with no load I think it would be fine.

For what it's worth:


I did a woodstove surround on 1/2" drywall spaced from the surrounding wall by 1/2" using 1" wide scraps of drywall every 8 inches.  used mastic and 1/4" tile.  No problems.
Later did a kitchen wall/backspash using tile, mastic on 1/2" oriented strand board, textured side out.  No problems.


I'm not sure in this application if you need even backer board.  I'd be tempted to make a frame that would hold the granite slab in location, with some bottom support, and keep it from tipping out with a bead of silicon at the top.