The point about ferromagnetic conduit _encircling_ the conductors is that the conduit is acting as the magnetic core of a transformer.
The issue is that the magnetic fields can induce parasitic current which can cause unintentional heating on top of the desired load current and its necessary heating.
There are two 'loops' that need to be considered: the loop of electrical conductors which can carry the parasitic current, and the magnetic coupling which can induce this parasitic current.
The encircling ferromagnetic conduit would act as a core to enhance the magnetic coupling.
The system described, with its ACSR cables and isolated phases, is more like an air core transformer. There will be coupling from the phase conductors to the ACSR messenger cables, and there will be circulating current in the closed ACSR loops.
But I repeat that I don't know how significant an issue this would be.