It is. Our anglo ears are unused to the 'ae' sound, so you tend to hear it pronounced as 'hyun-day', or even 'hyun-die'.
But Korean doesn't have dipthongs like English does; the sound 'day' would be pronounced separately as 'deh-ee'. Whereas the 'ae' is pronounced more like the 'a' in Mary.
That is the correct romanization of the name of the automobile company. A better question would be, "Why do we spell it as Hyundai and not Hyeondae?" From a quick online search, I do not have an immediate answer to that question, but I can think of two explanations that seem likely. 1) It could be referring to an obsolete system of romanizing the Korean script, where the results are spelled differently, but otherwise come out identical. 2) The English spelling of the company's name may be due to the phenomenon the English language experiences called anglicization, where a word's spelling is modified to make it more English-like. I suspect that both are the reason that we spell it as Hyundai and not Hyeondae, and as a matter of fact, according to the lesson's "Tips and notes", "proper nouns and words that had ... entered English [before the 1990s] often make use of one of the older systems" for romanizing the Korean script. Hope this helps.