Anas bin Malik said, “The Prophet (p.b.u.h) and Zaid bin Thabit took their Suhur together. When they finished it, the Prophet stood for the (Fajr) prayer and offered it.” We asked Anas, “What was the interval between their finishing the Suhur and the starting of the morning prayer?” Anas replied, “It was equal to the time taken by a person in reciting fifty verses of the Quran.”
So about 8-10 minutes, they waited for Fajr. Maybe they finished their suhoor early and it wasn’t time for Fajr yet. Or maybe they were preparing for Salah (doing wudu), or making du`a, or just having a conversation. Could be anything.
Point is, they waited for Fajr after suhoor.