“you will be as one of the godless fools in Israel.” This is true, but it was also an attempt on Tamar’s part to get away from Amnon by getting him to stop what he was doing by realizing the consequences of what he was doing.
“for he will not withhold me from you.” It is very unlikely that Tamar was being serious when she said this, but was rather trying any tactic she could think of in the moment to escape Amnon. The Law of Moses forbid marriage between half brothers and sisters (Lev. 18:9; 20:17; Deut. 27:22), and thus it is unlikely that David would consent to Tamar being with Amnon, especially if Tamar did not want it.