I'm no expert, but I think it's fair to call a boiler failure an explosion. you're going to have a ton of steam pressure in a confined space. Not like a gasoline or dinamite explosion, but still a hell of a boom.

although maybe it did have a special emergency release valve that was sealed with a weaker metal or something so it couldn't get enough pressure to catastrophically fail.

I'm skeptical of this claim at face value "impossible for it to blow up" but people are smart. it's probably true in a bunch of contexts.