Eating grapes is done in the front part of the mouth with the front teeth. Many foods are in the back. I suspect that it is a good one to add to a diet. Strong pressures between the front teeth can be painful... slow adaptful changes over time in a diet may be best! Grapes are $$$, so growing a vine of a type you like can be helpful to budget! Grapes are what many people ate years ago & turned into wine. This kind of stuff makes a lot of sense. Raisins are more with the back teeth - grapes with the front.
I suspect it to be a good food supplement to AGGA treatment. Keep a good posture when eating, especially.
Safe food choice for posterior open bite and desired forward movement.