I think the main message here is that...
Black people during that time (and maybe even today but that's another topic for another day) should question whether it's worth fighting, defending, and helping a country push its agendas/patriotism, etc when that same country doesn't care about you and only tolerates you because of the laws (Civil Rights, Brown vs Board, etc) put forth that somewhat prevents potential harm, physically and mentally.
It's not about joining the Chinese, Japanese, etc to go against America but rather staying out of the fight altogether and watching on the sidelines (basically the “no dog in this fight” perspective). If anything you could make the argument they were trying to give Black people a hint by saying why risk your life for THEM (whites, racists, etc) of all people after everything they've done to you and continue to do, and maybe instead prioritize yourself and those who look like you.
From that perspective, that's just general common sense.