"New" is a relative term. Last weekend I had a relatively new Impala (2015, >3k miles) but it was the old styled "fleet" Impala which is still in production. It was tagged as YF
I am not sure about Hertz but at Budget the new redesigned Impala that is available on the consumer market now is tagged premium alongside the Ford Taurus!
I wouldn't be surprised if Hertz is keeping the old fleet versions in F and the new fancy ones in G.