Top Ten Tanks

[Imp]

As I say no expert but knew someone that was & used to use diesel a lot in 50 gallon drums.

On these 2 points

It's also going to fireball (like in the movies, which is far less energy efficient than a proper explosion), drenching the vehicle in heat and smoke (all of that unused energy) and marking its location for the next 5 minutes (this is bad).
&
As mentioned, diesel just sitting in a can usually is a poor explosive. It refuses to vaporize properly under 120F/49C. (Diesel engines reduce the effective flashpoint via compression, but used to be notoriously hard to start in cold weather for this reason.)

My thoughts
firstly as you say acts to a degree as spaced armour.

It would be a shaped explosion the round hitting it but more importantly the speed of the plasma would massivly compress the diesel easily hitting flash point then being vaporised.
The blast would be centred on the area of pressure so the explosion would be aimed mainly at the plasma jet taking energy out of it & hopefully distorting it.

Any fuel that ignited did not blow up due to compression by the jet, stuff on edge will be vaporised before ignition. Diesel further out that misses the blast will ignite giving your fireball.
From a protection point of view this is fine as would not effect the plasma cone & is ignited by heat alone. The plasma is nearly twice as hot as the sun 120F???? thats why it melts through armour needing vapour to ignite easily probably not an issue.

Yes I have dirty black smoke need to change the filters but it will burn out quickly & we are still in one piece.

This would have little effect on the crew or vehicle the part exploding vs the round would send most of its energy away from the vehicle.
The part that ignites will be a minor blast in all directions & take the path of least resistance also mainly blowing outwards.
Actually placing blocks of explosive on the tank to do this has been around for years using jerry cans not as scientific or controlled but can see why someonethought to test it.

Same principal is used to blow up a drum of diesel stick the TNT where you want to aim the explosion.