They announced publicaly last Wednesday (27 May, 2020), but didn't say when they started alerting publishers. However, at the time of the announcement, there were already 2k+ games that devs had opted-in, which would likely mean a few days or weeks.
That one game has been opted-in and one hasn't for the series is confusing to me but it hopefully was just a timing issue and this coming Thursday will have Deadfire back on Geforce.