FX reserves tell us whatever every country's FX tell us. That countries have forex, gold and bond reserves to protect the value of their currency. It can also work in the opposite direction. India and US keep large sums of gold. Greece kept large sums of US govt. bonds (until 2008 market crash at which point they found out this was a bad idea).

Japan's Abe is famous for auctioning huge reserves of JPY in order to reduce the value of their currency and stock up on USD. Reduced currency value means their exports became temporarily cheaper and this way they got an artificial boost to their production industry.

This is a complex subject and doesn't show that "country X is the puppet of country Y", it rather shows how they like to strategize for their own benefit.