If you take a look at the wiring diagrams, you can see that the “ECON” button on a non climate controlled car is actually a physical switch that connects directly to the ECU.
On a climate control car, this connection isn’t present and the state of this button is instead communicated over CAN bus.
It is true, there is no “coding” to tell the ECU if it has climate control or not (there actually may be, I need to research more). But the ECU is still “aware” via some internal flags that are part of the “map blocks”.
For the heater to work, 2 flags need to be set correctly. One flag tells the ECU to decode the CAN frames from the climate control. The other is a switch that moves the auxiliary heater inhibit source from a pin state “physical” to a memory location: That of the previously decoded CAN frame.
I imagine the default values for these flags is “no climate control” or at least the “physical” auxiliary heater inhibit button, and the calibrator who setup these ECU’s in the early 2000’s just didn’t bother to change them for map block 5. This is the reason that either coding setting will work for those who have none climate cars.
I’ll add some more details and references to support this later