You are correct, "Callous though it may seem" does in fact equal "Though it may seem callous". The form you encountered it in is idiomatic (it's an expression used exactly in that word order and no other way) and is in common use amongst native speakers. This specific phrase never feels strange simply because it is an idiom.
Without it being an idiom, the construction of the phrase would still be valid, but feel quite archaic. You'll find similar constructions in the King James Bible, which is the source of a lot of the English language used today. For example, "Happy is the man who..." instead of "The man who ... is happy." It's valid English, but feels old. People sometimes write like this to deliberately make something sound old, or at least overly formal.