Comment by kube-system

But it is strictly correct. Deflagration is the definition of low (i.e. subsonic) explosive.

Combustion is a broader term — combustion just refers to burning. And the reason it is called an internal combustion engine is to contrast it from its predecessor combustion engine designs - the external combustion engine.

More precisely, the type of combustion happening in an ICE engine is (low) explosive.

https://en.wikipedia.org/wiki/Explosive#Low