The act does not require the US to use metric in any capacity. It is optional. If it did kids would not be taught using the imperial system.
The US uses customary measures. Imperial is the old UK system. It has some important differences like the size of a gallon - and perhaps most importantly if you like beer: the size of a pint.
There are a lot of situations in the US where using dual measurements, or just using metric is required, like food labeling. Using customary units is optional in many cases