Potentially the crude has weathered off so much over the years that it's no longer degassing, and less scientifically I think if the tank vapours haven't found an ignition source yet, they probably won't.
However corrosion never sleeps. The cargo tanks don't leak straight into the sea; you have ballast tanks too. If I had to guess, the next threat might be a leak into the pump room resulting in fire.
But ships have a periodic inspection regime under that they call "class rules" - this ship hasn't been in class for 5 years.
That is an insane period of time to be carrying hydrocarbon while being out of class.
There are hundreds of threats to this tanker and it is difficult to predict how it could fail, including complete loss of the vessel.
Explosion in the first place requires enormous amounts of oxygen. Even with inert atmosphere replaced with air, the volume is just too low.
The initial primary explosion could cause significant enough destruction to the ship that it acts like the “scatter charge” in a thermobaric weapon, spreading the undetonated oil sufficiently into the air that a larger percentage of the onboard oil is detonated as part of a “single event”…
However the more likely and no less disastrous outcome is the primary explosion of the fuel vapour damages the ship without sinking it (the explosion will be centred on the upper portion of the storage tanks) most likely blowing the top off of one (or more) of the storage tanks and potentially damaging the structural integrity of the ship without sinking it. An extremely likely outcome from this will be ignition of whatever remains of the oil in a burst storage tank, (which even if not “burst open” will likely have been structurally damaged so that fresh air can get in) leading to a fire onboard and due to the established lack of maintenance, that fire has a high likelihood of spreading through much of the ship (possibly aided by any oil movement induced by the primary explosion) which will heat up any undamaged storage vessels and turn them into (depending on the state of the pressure relief valve maintenance) potential pressure cookers full of fuel or sources of further burning fuel vapour…
So I don’t think they were trying to necessarily mislead as much as simplify the scenario for the audience. Most people don’t find it interesting to dissect cascading failures and map out the energetic potential effects. Most people would be fine saying “it blew up” even if it was technically two or three individual explosions interspersed between hours of being on fire before one final still in tact hot tank of oil went off like a pressure cooker turning into a fuel air explosion that shredded the ship into shrapnel.
Oil doesn't detonate, even if scattered by an explosion. Fuel-air explosions require very specific conditions it's entirely unlikely those conditions could be met with crude oil. Thermobaric weapons use chemicals like ethylene oxide that are: 1) highly volatile, 2) easily dispersed [no thick like oil], 3) have a very broad air-fuel mixture range which is flammable.
Could the vapor explode and spread burning oil everywhere? Sure.
Could it explode, distributing microscopic oil particles in such a way to sustain a fuel-air explosion? Highly unlikely.
Regardless of the failure mode, there’s probably a few that people haven’t thought of yet.
Liquids can't burn in air, only gasses can.
(Technically it's possible to make a liquid oxidizer and evenly mix it into the fuel, in which case it should be able to burn, but that's not something that can happen normally as there are very few room temperature liquid oxidizers. (See https://en.wikipedia.org/wiki/Triethylborane and https://en.wikipedia.org/wiki/Triethylaluminium ))
More importantly, high enough temperature and if you're unlucky the metals in the ship turn out to be thermite...
So that scene in "The Equalizer" when Denzel Washington blows up that oil tanker would in reality become a massive oil spill, or fire?
That would have been a strange plot twist, the equalizer himself being the cause of a massive environmental disaster :-)
Remember when the protagonist in Surrogates caused sudden, global economic and infrastructural collapse, possibly killing billions of people off-screen, just because he wanted to have richer sex with his wife?
Who can blame him :-)