You mean they
didn't have David Bowie in the Middle Ages? I call shenanigans! Bowie is timeless. Really, though, if you want to get technical, the sweeping orchestral soundtrack you hear in most medieval-esque movies isn't accurate, either.
I thought it was a rather fun movie. One of my favorites. I wouldn't put too much stock in the historicalness of it, but it has good entertainment value. The real Ulrich was an Austrian/German ministerialis who, like you mentioned, was around at a much earlier time. I always just kind of assumed William had heard the name somewhere before and just decided to take it up as his own for the tournaments. At least, that's the story I always went with. So it's not so much an account of Ulrich von Liechtenstein's achievements as it is of William Thatcher's achievements.
Also, I think at that time "Jocelyn" was still predominantly a masculine name, so I always kind of giggle a little at that. Guess it's along the lines of a boy named Sue.