The FSI 3/5 entry is extracted from the flag state (Liberian)
investigation report, which we are not allowed to see.
It is not very clear; but 15 years after the Mactra et al,
the ship was almost certainly not inerted.
Charterers felt that IG might contaminate clean products,
so IMO did not require IGS on products tankers below 40,000 dwt.
and few products tanker owners installed it.
if you need to enter the tank,
gas freeing by ballasting is safer than simply venting the tanks,
But the problem is you are pushing a flammable vapor
out on deck,
The real fault here is a regulatory system
that took close to 20 years
to impose inerting on (almost) all tankers.