Ahhh ... I see your point.
The PDF rules say "the leading player(s) may not play a 6" then refers to a footnote that says:
Typically the innermost chariot when several are neck-and-neck in a curve — in a straight stretch, consider the upcoming curve. It may be that several chariots are all “leaders”; in this case, none of them can play the 6 card.
I would also be confused by this, and it seems to have been discussed here: http://www.boardgamegeek.com/thread/35966
I've only played the new edition which avoided this issue by simply saying that "joint leaders can't play a 6".