This statement is false. At the end of 1939, belligerents were not barred from buying arms in the United States.
1939 was in range of a time in which the USA desired for their allies to support their neutrality. They didn’t want to get in any wars, they wanted to stay out of foreign affairs, and they didn’t want to have to pick sides. Although transportation of arms to belligerent ports were banned at the time, there wasn’t a specific law that prevented or dissuaded them from buying arms; in fact, it was just as normal as a civilian purchasing one.