I still maintain that Captain Marvel had reverted back to his mortal/Billy form when the bomb actually detonated. It was shown a few pages prior that if the object he "Shazam'ed" was TOUCHING him as the lightning struck, it would revert him back into Billy as a result.
Here he 'spam-Shazams' Superman with lightning, and is still in his God/Marvel form:
But when Superman grabs him just as the lightning strikes, *poof*, he reverts back into Billy:
This is important because when he 'triple-Shazam'ed' the bomb, he was TOUCHING it:
...So the same pre-established rule would have logically applied: he simultaneously reverted back into Billy just as the lightning strikes hit the bomb -- ergo he was mortal when it detonated.
Not only is this a completely logical assertion(I really don't know how it can be viewed any other way, tbh), but it also makes the most sense. I mean, Captain Marvel was shown and described as Superman's physical equal in EVERY way... Yet he gets vaped/atomized by an explosion that Supes essentially tanked? No way... He was mortal/Billy.
...IMHO. 🙂