Others commented on this. I actually agree that given the entire philosophy of utter depravity of man, Luther's comment can be better understood, as Harley explains. But what really is happening is that the fact that Luther could make that comment at all shows the bankruptcy of Luther's theology.
As I googled the "sin boldly" quote, I found several embedded in actual sermons by Lutheran pastors. I dod not see any assertion that the quote was somehow inaccurate or out of context. The reason I chose a Catholic website to give a link to is because there were several corroborating quotes there, all rather shocking to an unbiased reader, and they were all meticulously sourced.
It has long been my experience that quotes on posts like this are often taken out of context . I generally perfer to go to the source. If that is not possible I go to the website that generally support the person. You will find that when I discuss Catholic doctrine I usually refer to Catholic websites. Also with historical documents such as these, it is important to understand the background with which these are written.
Luther was not a rabid scholar. He was highly esteemed by the Catholic Church so long as he agreed with the Church's position. It's only after he broke with the Church that he lost all sense of reasoning. Catholics do not look very favorably on Luther. It is unlikely a Catholic website would give an unbias look at Luther's quotes.