That's actually not true. Project Black Flag was being worked on as far back as July 2022 - at least 6 months before the OGL situation.
Please take a look at the full text on the Project Black Flag FAQ page:
Project Black Flag - Kobold Press
In particular ...
So Project Black Flag isn’t a new game?
In July 2022 the Kobold Press team started work on Project Black Flag. Then, In January 2023, as our biggest fears were coming true, Kobold Press knew that Project Black Flag’s timeline had to change. Quickly.
In our initial announcements, it was necessary for Project Black Flag to be a new Core Fantasy Roleplay system. However, once SRD 5.1 was released into Creative Commons, it was suddenly clear that we could build on the same foundation that so many people already know and love. As a bonus, Project Black Flag can streamline the new building while ensuring a thriving future for 5th Edition.