It was an explosion by the first definition - it says 'as is'. The casing contained the explosion.
We shall see
It is incredible the noise and visual effects a mere "surge" can have, and cause little/no damage.
Was the runway closed for sometime afterwards requiring debris to be removed? If not, then to me anyway it was not an "explosion"
If we want non specialist terms, then "backfire" might be more appropriate - we shall see (or maybe not - unlikely any AAIB follow up).