Yes, it was an odd year. (Though this was my 6th harvest, and it seems like every year is a new experience...)
Grapes were moving along nicely and then all of a sudden... 20 brix. Yes, this is fine. No need to ripen any further.
I think we brought our* grapes in a little early in some cases, though probably a good decision since there has been some intermittent rain during harvest season, further complicating the ripening/brix/pH dynamic. It also seems like buffer capacity was a bit lower than past years, so I'd advise caution if you are inclined to add acid.
It sounds like your grapes are in a good place, though. What varietal?
* not my personal grapes... the winery where I work
