I always use the engine brake on an F650 ,which has a caterpillar engine. It recently blew up and I've been told that this is the cause....is this even plausible?
Yes it is possible - but without much more information it wouldn't be possible to state that that is what happened in your case.
If you were coasting at high speed, with the engine idling, if you suddenly let out the clutch to use engine braking, the transmission will drive the engine so the loads aren't the same as when you are accelerating. The engine should cope, but a possible failure mode is that the timing belt could slip/snap/jump and if that happens, your engine could fail catastrophically.
That said, timing belt failure can happen under acceleration too, but the only difference here is the sudden high load.