Had the battery been on the charger, generating hydrogen gas where a spark from a bad battery cable connection could have ignited said gas?

Yes, Thank you Bill

Bill, isn't that the reason that it is advised to ground the negative cable to the engine block when jumping a battery?

Yes, using the engine block (or other substantial ground) AWAY FROM the battery to connect the negative clip of a charger or booster cable is recommended/advised so that any inadvertent spark/arc is less likely to ignite the gases hovering around the top of the battery.
BUT, the attaching location away from the battery (for the negative clip) is only half of the procedure. The other half of the instructions is that this connection be made as the LAST connection in the sequence of attaching a charger or booster cables...............and the FIRST connection to detach when removing the charger or booster cables.

It doesn't help at all if you connect the negative cable to a remote ground and then make a last connection of the positive cable going directly to the battery post.

Sequence of events is as critical as where you make your clipping point.
The image below gives a visual explanation. In the case of battery chargers simply consider the "good battery/donor vehicle" as the charger when charging the battery while it is mounted in a vehicle. When using battery chargers, ALWAYS make certain the charger is not plugged in to house current and that all the charger controls are in the OFF mode BEFORE you connect the clips to battery.

From what I read in the OP he had the batt out and reinstalled it or had one or both cables disconnected. In this case make sure the batt posts and clamps are cleaned w/ a batt post/ clamp cleaner or wire brush before reattaching.
Mike

Yes, that is true, but it never hurts to reiterate a safety procedure when the opportunity appears.
Sometimes a battery will explode in what seems like an "unprovoked, spontaneous explosion".
These events happen because of an internal arc inside the battery and usually happen in batteries where the electrolyte level has fallen low enough to expose some of the lead plates instead of them being submerged. When some of the plate material is exposed, there is an increased likelihood of an arc (even a microscopic one) flashing between the adjacent plates........especially if the plate separators are coated with sulphate debris and crumbs.
Even the smallest arc will ignite the explosive gases in the head space above the plates.

I have a reminder on top of my charger: RED CABLE FIRST ON, LAST OFF