Given the outcome of the hazard was fire and/or explosion, who on earth would classify a fire and/or explosion as “marginal” severity? That’s insane.
Another possible issue that springs to mind is that I read somewhere that those undertaking the safety case were unaware that the SCP was operated in flight - hence this misunderstanding could have had a bearing on their (mis)assessment of the risk. As such they would have incorrectly assumed there was no credible source of ignition in flight in that area.
Alternatively even if they thought the SCP was on, they may have incorrectly assumed the SCP piping was not infact a significant ignition risk.