Yes, as everyone is stating, a spark igniting hydrogen gas could cause an explosion, causing injury from small parts, sulfuric acid or both. Very nasty. Hydrogen gas is a byproduct of electrical energy created from a chemical reaction of lead plates submerged into the acid and water. Sparks can happen internally, too. Like when the lead plates warp from water evaporating out. This is why some are cynical about 'sealed' cells on an unmaintainable lead acid battery. Spark wouldn't necessary come from jumper cables or charger cables in this case, though, but rather simply attempting to start the vehicle.

It seemed like there were a couple of specific questions you had that I will attempt to answer.

Don't attach negative clip to the neg terminal on the dead battery unless you want the car's wiring removed from the battery. Attach to clean, unpainted part of frame or engine block bolt. This will keep any possible spark or spark arcing away from the potential gas. The contradictory part of whatever website you wrote from in your original question post was obviously either a mistake or the part where it spoke of attaching to the negative terminal was describing the process of jump-starting from a charged battery to a dead one. In that case you always attach dead-side first, with negative clamped to frame or engine, but the positive and negative attached to their perspective terminals on the charged battery (on the other side of cables). Again, the exception is if you remove the car's wiring.

The reason you go from dead to charged is to reduce the risk of spark. You can't take the risk of spark away completely, which is why it's better to attach the negative clip away from neg battery terminal. Same principal with a charger. Don't plug into power until clips are clipped. Just like going from dead to charged when charging from another battery, don't plug in charger until clips are on.

Another question I think you had was if there are other types of automotive batteries that don't have this risk. The answer is that unless it is lithium ion (or similar) batteries found in hybrid/electric cars or sometimes found in an emergency kit, they will ALL be some kind of lead acid type, even though there are a few different kinds of lead-acid batteries in use. They all hold the same risk.

Here is a link to a credible source regarding a study that took place with lead-acid batteries and explosions. It also includes all the other information I stated above about safety measures like hooking the negative to frame or block instead of terminal and the reasons for these safety measures: