The phrase "frame buffer" refers to a temporary storage area in a computer's memory that holds the image being shown on the screen. It helps in quickly displaying visuals and allowing smooth movements on the screen. Full definition
He also assured the system will run games at native 4K and will support a 4K frame buffer. (thetech52.com)
But, I totally agree that it was extremely misleading and they should differentiate between true and frame buffering resolutions in the future. (n4g.com)
However, during the GDC 2018 Optimizing UE4 for Fortnite: Battle Royale presentation, Epic showed a slide indicated that they are targeting a full 4K (2160p) resolution as the max frame buffer soon on the console. (wccftech.com)