Slightly pedantic mode ON ... (trying to explain how it works mind !!)
Talking about "turning ECO ON" is a bit of a misnomer .... it really is "selecting ECO mode" .. in ECO mode the ECC continues to work and attempts to regulate temperature BUT the compressor of the air conditioner no longer runs ( hence ECO for economy ....)
The effect of the compressor/air con working is to cool the air considerably. Cold air cannot hold moisture (which is why we get fog on winter mornings) so the air dries out. It is then heated by the matrix if required to give warm, dry air. This greatly reduces condensation inside the car.
With ECO selected you still get warm air, but it is "undried" ... so holds more moisture to condense on cold surfaces.
With "recirc" selected no "fresh air" enters the cabin .. so any moisture already there satys trapped, and because human beings breathe ( I find it a useful attribute

) the driver and all pasengers slowly but surely increase the amount of moisture in the cabin air (humidity) the more warm wet air .. the more condensation on cold surfaces.
So, to get the "best" demisting ... air con running, ECO off, Recirc Off, rear vents clear and a warmish temperature selected.
HTH
