The "protection" they were referring to was from stones. Insoles are pretty thin, so the stones can be felt through them, sometimes painfully.
Frankly, I'd prefer the sandal alternative too, I just didn't have any idea how to approach the task. I didn't have a lot of spare pad material, so I wanted to get something that would work the first try. I hope you post your final sandal design.
I think you'll enjoy them. For light weight AND comfort, I haven't found anything that comes close.