I have just done a bit of testing with people I know are on the 1939 Register on FindMyPast.
When Ancestry say they have not made it searchable yet, I think they mean you are not yet able to select someone in your Ancestry tree, click search, and locate them in the 1939 Register (as you can with census records etc.) However, if you select 1939 Register from the Card Catalogue and input the name/details of the person you are searching for you will find their record.
Edit: Ignore everything I said

, it is searchable from selecting someone in your tree