David was not the only one who did not know Avraham and Yitzchak. In the whole book of Shmuel, Avraham and Yitzchak are not found: The author of Shmuel did not know them, just as David did not know them. Because, they were not invented yet, or they were in the province of others.
The new Torah that will come out of Zion, will be based on the truth. Nothing else will save us. The goyim will come to chase us, also in Israel, until we will say the truth. Not stories, like the religious are wont to do. Not nothing as the Chilonim tend to do. We have to tell the truth. Until that time, G-d will not hear us. The seal of G-d, the G-d of Israel, is Truth.
