like after all I have looked up about the differences between a coupled inductor and a transformer it seems like it is just down to application.
so like what is stopping me from using something like https://www.coilcraft.com/en-us/pro...rs/1-n-shielded-coupled/lpr/ua7868/#UA7868-AE
as a to reduce voltage like a transformer but NOT in flyback topology?

I know I am missing something small but significant

 

The specific coupled inductor is designed as a energy storage device in a current pulse based circuit, I would expect it to be a less efficient for non-pulsed energy transfer than a device designed for AC sinewave transformer transfer action.

 

 

I know I am missing something small but significant

Yes, it's significant but not small.
It's the inductance of the inductor.
It must be high enough to keep the magnetizing current below the saturation current of the inductor.
Since that current is directly proportional to voltage and inversely proportional to frequency, the lower the frequency the larger the coupled inductor (transformer) to get the required inductance for a given voltage.
That's why 50-60Hz mains transformers are much larger for a given power, than a switch-mode supply which operates at 10's of kHz.

 

o like what is stopping me from using something like

I have used coupled inductors as transformers, from Coilcraft, and it works. I would use that one at 1mhz not 60hz. It is only good for 300V isolation so not good for power line isolation.