This appears to be Samuel Kwok's website and he is respected and legitimate. I see what you mean about the wording on the website, I suspect what he means is that he teaches "Ip Man Wing Chun", which he does. There are credible wing chun branches that don't roll up to Ip Man, so technically anyone in a lineage under one of Yip Man's disciples or sifus is doing "Ip Man Wing Chun" ... officially.
Whether or not you should supplement your training with a DVD is another question. I strongly suggest that you talk with your sifu about it and respect whatever his position is. I can always tell when someone has been on YouTube. It's not always the best thing for their progress, regardless of the quality and source.