That statement is not entirely accurate. While it is true that the president nominates ambassadors, the appointments must typically be confirmed by the Senate before they can officially assume their roles. The requirement for Senate approval is outlined in the U.S. Constitution under Article II, Section 2, Clause 2, which states that the president "shall nominate, and by and with the advice and consent of the Senate, shall appoint... Ambassadors." So, in practice, the president does select ambassadors, but their appointment must be approved by the Senate.
