Quite correct as noted above.
You must complete the Flight Review as that effectively starts the clock ticking for the validity period of 24 months for the US certificate. It is irrelevant as to whether you flew the day before in your own country on your own national license.
Also as stated the Flight Review is a MINIMUM of 1 hour ground and in the air and that is for a seasoned US pilot who knows the regulations inside out.
Anybody completing the ground in an hour who has never studied the FAR/AIM did not receive an adequate ground review - in my opinion; and any self respecting Instructor shouldn't put his name to it.
The flight portion might be squeezed into an hour but you should assume at least two - again in my opinion.