"I have noticed they are now laying a sheet of plastic beneath...

"I have noticed they are now laying a sheet of plastic beneath the pour."

Interesting, maybe those steel fibres rust out quicker (larger surface are to volume compared to steel mesh ) from moisture ingress so an impervious plastic membrane is needed , ?
If so personally looks like steel fibres are a disaster in the making once they rust out in future years, as they will compromise the structural integrity of the concrete as they slowly rust out creating spaces for water to freeze and thaw and cause spalling .
At least with steel mesh the mesh is in the middle of the concrete so further distanced from the surface for moisture ingress compared to steel fibres which by design are mixed throughout the concrete .
CNTs are basically inert so not an issue at all .
If those steel fibres rust out then there are going to be some ginormous legal ramifications for the manufacturers , like James Hardie had with asbestos .