'Enamel' in this case is a matter of semantics...
The enamel you see on most cast iron cookware sold today is actually some sort of metal alloy, which does the whole expand/contract thing pretty well. Back a century or so ago, it was a powdered glass glaze (what we assume is enamel), but that's fallen into disuse. I guess it could still shatter if the underlying iron was stressed in some way.
From everything I can tell, the treated interiors of most tetsubins are made of a sort of porcelain enamel, which will crack if directly heated.