The number of yuan per dollar getting lower does mean that the yuan is becoming stronger relative to the dollar. (It doesn't mean the yuan is becoming stronger; it could just be that the dollar weakened.)

But that would be called a "rise" in the yuan. A "fall" in a currency always means the opposite, that the value of the currency has dropped. In the usage you're questioning, the yuan "fell" to 7+ to the dollar from its previously higher value of 6+. It's not a reduction in the number of yuan per dollar.

It helps to think about exchange rate in reverse direction: Previously 1 yuan ~= 0.166 USD, now 1 yuan ~= 0.142 USD, so its value clearly fell.

Thanks for the explanation!