I’m a bit confused on why 本当に is considered a jukugo word. From what I understand, jukugo words are words that don’t have any hiragana in it. Is it just cause its a regular jukugo word (本当) with a hiragana character (に) tacked onto the end?
The definition of jukugo that matters here is “a word composed of more than one kanji.” Which this is. The presence of hiragana is not a part of the definition of jukugo.
Though to be fair, it’s really just 本当 plus the particle に, but they get used as a standalone expression enough that you might call it one “word” in the English thought process.