Joe Biden won the 2024 U.S. presidential election and was reelected president, defeating Donald Trump. Multiple major outlets report that Biden secured both the popular vote and the necessary Electoral College majority, confirming his victory.

Because the prediction was that the border wall issue "could cost him the election" (i.e., lead to him losing in 2024), and Biden in fact did not lose the 2024 election, the prediction is wrong.

(If one interpreted the statement more loosely as merely "this will be a major liability" without the explicit electoral-loss claim, that might be debated—but the quoted line is specifically about costing him the election, which did not happen.)