I wouldn't think it's 28mm thick I would be more worried about the ply mosture and the biggest problem with thin ply is delamination unless you fix it with adhesive but then you have the same problem as fixing tiles to (using the ply like a tile)Do you really think it would be necessary to use cement board for a tiled work surface? There's not going to be any deflection issues as it's not likely to be walked on and the MDF worktop (if it is actullay worktop) should be at least 28mm thick. Screwed down to rigid carcasses. I would say the ply would be sufficient but prime before tiling.
tiled 5 stainless steal man holes today. used ardex waterproofer kit first and then mapei keraquick. PERFECT