In reply to Noxeal:
As La Benya says the plus now signifies a different grade. Though originally in France (where this grading system originated) the plus was just meant to mean exactly what you say: hard for the grade.
However, as the difficulty of all climbs is on a continuum, there seems an inevitability that once used consistently it has to become a different grade. The problem is where do you draw the boundary lines from one grade to the next. If say a plus grade was to represent the to 50% of routes in a grade then it is effectively a new grade.
Another grade that is sometimes used is the infamous slash grade. For example 8a/8a+ or just 8a/+. It's not a proper grade but sometimes used where the the first ascentionist can't decide which of the two grades it is. Likewise Ondra's grade suggestion is that his route is high in the 9b continuum of difficulty.
I'm a little surprised at Ondra's suggestion that this is a hard 9b because it didn't appear to take him long to do. But he usually seems pretty careful with grades so I guess time will tell.