I'm not sure what the 3.4 means for the accent panel. I don't think it means 3.4 inches or 3/4 inch because they are illustrated as being thinner than the plywood which is 5/8" and they feel pretty thin. Since they are using 1x2s for cross support I would think there is less than 1/4 inch behind the plywood on the bathroom side since the plywood is flush mounted on the dining facing side.
Technical support also said that the tv backer board should support 20-25 lbs of weight. In looking at tv and articulating wall mount weights it looks like 24 inch Led tv with articulating wall mount is around 20 lbs. 28-32 inch tvs look around 16 lbs and articulating mounts around 8 lbs which is a little to close to the support max for me. I would think a non articulating wall mount would weigh less.
I share your nervousness about drilling and screwing into that wall.