I think it was a warning. 

No idea if he interpreted it as such though. Maybe he thought it was re and Japan would be defenseless without the US, maybe he thought it was sabotage and Jap ships would suddenly start getting blown up by his "ally". Maybe he feared for his life. In any case it was a difficult position to be in for sure. 

For future attempts at de-Americanization, it raises the fear of "are we going to get invaded by China if USA leaves?" They'd need to build up their own defenses first, and USA has every incentive to block any attempt at that. It's a tough nut to crack.