I was wondering if someone could have a quick look at my simplified set up.
A box with mtlx transparency set to full(Base diffuse set to red), sitting in front of a sphere(base diffuse set to green).
I don't get a transparency happening.
I tried a set up with principle shader (in the network box), and can get the transparency.
I must be overlooking some simple setting.

You have Metalness set all the way to 1 on your mtlx material. If you turn it down you will see transmission return, and at 0 it looks close to the principled example.

Thanks for pointing out where it's coming from.

However, although I don't have it set in my file, one can still have 'metalic' set to 1 in the principle shader and still have transparency.

I wonder if it's just a matter of the stage of development or that fundamentally at the core each functions differently and that to achieve similar renders in any regard one just has to approach it differently for each shader and not expect some kind of one to one correspondance.

BabaJ
I wonder if it's just a matter of the stage of development or that fundamentally at the core each functions differently and that to achieve similar renders in any regard one just has to approach it differently for each shader and not expect some kind of one to one correspondance.

No, it's a matter of the two shaders not being the same. Read the whitepaper on the standard surface learn the logic.