Is 'Pious' an antonym of itself?

One reason it can happen is topical shift, and especially the processes of amelioration (more positive meaning) and perjoration (less positive meaning). The meaning of a word can drift over time to the extent it becomes an antonym of itself. If you have access to the Oxford English Dictionary online (likely available at your friendly local university), look up a word with a long history, like the adverb/adjective "nice": over the years "nice" has meandered from "foolish, ignorant" (ca. 1300) to "wanton, lascivious" and ameliorated into "extravagant, ostentatious" (mid 15th. c), to "finely dressed" and went sideways and down a bit to "scrupulous, punctilious, difficult to please" (18th-19th c.) before easing back to "refined, cultured" and even "tasty, pleasant." And even now it has a sarcastic/dampened/ironic meaning. Nice ain't great, but it doesn't mean dumb either.

Another spot where this kind of shift comes to mind is the role generational dynamics play in slang — especially words meaning "cool." Teenagers think their parents' lingo is lame, and so words such as swell, dandy, gnarly, rad, and lit all shift in meaning, even within a generation. One couldn't assume that today's whippersnappers think that someone dressed up all dandy would be totally lit.