I think that there may be a difference between the technical and popular descriptions. The 'De Wilde' definitely contained no explosives in the accepted sense of any of the various HE natures (TNT, PETN, Hexogen etc), the content was purely classified as 'incendiary'. Technically, the difference would have been in the rate of burning of the material: gas from an explosive expands extremely violently causing a shattering effect (brisance), incendiaries burn relatively slowly even when they appear to burst instantly into flame, and have no brisance.
Tony Williams: Military gun and ammunition
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