[dragonwriter]

> I had to look up "penal offense." The relevant definition seems to be "liable to punishment."

You seem to have looked up “penal” alone in a general-use dictionary, rather than the legal term “penal offense” in a law dictionary. Unsurprisingly, the treaty, a document of international law, uses a legal term of art in (one of) it's legal sense(s). The term generally means “an offense punishable by law”, but is more specifically construed in a couple different senses, narrowly as synonymous with criminal offenses, but sometimes more broadly incorporating also offenses which are not criminal but include an exemplary / punitive sanction as well as (and particularly not tied to) any compensatory one.

Termination of employment is not a sanction (punitive or otherwise) by law. (Well, in the case under discussion; it can be a legal sanction, e.g., under military law.)