There is actually no suffix!? Or do you mean the Harakat ( َ / ِ / ُ )? Those are no suffixes but short vowel marks. In this case I guess they want to make clear that the pronunciation is "Dawuud". However, I don't understand what kind of name دَوود is supposed to be anyway. For David I know دَاوُود/دَاؤُود/دَاوُد in classical Arabic or دايفيد/ديفيد/دافيد when talking about the English David (like e.g. ديفيد بوي - David Bowie). I have never read دَوود before. But maybe some native can help!
Yeah, all courses start out with several mistakes. It's challenging enough to create a course that teaches a language close to English, and Arabic is even more challenging, not only because it's very different, but also because of all the different variations and lack of experience with diacritics, even from native speakers, because they're often dropped.