The check valve only applies to inline trichlor or bromine chlorinators, more so for the acidic water conditions they create, not so much the chlorine. A salt cell is wired to the pump relay in automation systems so the cell has no power if the pump is not running. Where automation is not used, the cell has a flow switch that will not allow the salt cell to operate if flow is not detected. This is to prevent an explosive condition from a buildup of hydrogen gas, not so much excess chlorine.
Since no chlorine can be produced if water is not flowing, there is no need for a check valve in that location.
Your plumbing setup is fine as is. I saw no mention of a check valve in their literature and the only thing related to use with a heater is to space the Intellichlor 3' from the heater outlet (page 5).