concrete caulk instead of grout?

No! its not designed for this application. Use the proper product for the proper application! Save yourself from the headache later! You need to realize that the grout in particularly made to adhere to the specific tile, just like the concrete caulk is made to adhere to concrete. Grout is also intended to help minimize movement in the tile. The floor does breathe and shifts some.. The ingredients in the grout are made for this. My guess is that the person only wants to use this because its in a caulking tube and he is thinking that it would be easier to apply--less mess, to the tile. I guess the concept is good and wish they did make something like this.