"Is it true?"
本当 is the noun and means "truth" or "real(ity)"
本当な is the adjective form and means "the true/proper/correct [following item]."
本当に is often translated as "really," but in the sense of "truly" rather than the sense of "very." In fact, "truly" is probably the more accurate translation.
so "kono ramen wa hontouni oishii desu" would translate to "this ramen is in fact delicious"(like if someone doubted it deliciousness) rather than "this ramen is very delicious"? or maybe the latter became popular and that's the message i would pass?
i know your comment is 2 years old, but maybe someone can help me
the difference between 本当な and 本当に is that the first is an adjective, meaning it can change nouns, while the second one is an adverbial form meaning that it can change adjectives, verbs and other adverbs. If you were to say 本当においしい you are changing the adjective with a sense of truly.
I think the original comment was trying to make a point that if you were to translate "very delicious" or "very +any other quality" to japanese you would pick とてもおいしい instead of 本当に.
In the same line of thought, I think you are missing a way to use truly in english, you can see for yourself here in these examples. Specifically: "a truly magnificent performance" would mean you are emphasizing the magnificence of the performance instead of proving someone that theirs doubts about its magnificence are wrong. In the same manner, in english, "truly delicious" is just making emphasis on the delicious part, the same happens in japanese as far as I know.