Public opinion had trended sharply against isolationism by late 1941.
As this summary of gallup polling shows, US public opinion had shifted dramatically away from isolationism and toward favoring involvement in the war by 1941, with the obvious final big shift in December after Pearl Harbor. Congress actually investigated whether the film industry was campaigning for US involvement by inserting pro-interventionist messages into their films. While the linked article makes the point that no Hollywood films before Pearl Harbor overtly advocated immediate US involvement in the war, that does not mean more subtle messages were not included in Hollywood films.
Anyone paying attention knew the US would be in it sooner or later.
While FDR was still maintaining the fig leaf of neutrality in 1940-41, anyone who was watching what was going on knew that the US had effectively taken a side and it would only be a matter of time before US forces were actively fighting. By 1940, the US had more than a million men under arms (and was drafting more), was quickly building a two-ocean navy, and was literally putting weapons into the hands of the British. US forces (mostly Navy) had already fought and died in the war on several occasions before Pearl Harbor. Further, Americans had volunteered by the thousands for the British and Canadian armed forces to fight before US involvement in the war. Some even went to fight for the Germans.
The long and short of it is that public opinion and the actions of both the US government/Military and thousands of individual Americans were all sharply indicating that US entry into the war was inevitable by mid-1941. Both isolationists and interventionists viewed the pre-war draft as a sure sign that FDR was getting ready to go to war. Jimmy Chan's line was probably completely authentic in the degree to which it reflected the thoughts and outlooks of young American men getting drafted in September 1941.