That interpretation is wrong. If a currency falls by 100% against any other currency, the currency is worthless.
On the other hand, 100 Yemeni Rials could buy one US dollar, but can now buy only
\[\frac{100}{280}=0.357\]
US Dollar. The Yemeni Rial has depreciated by \[\frac{1-0.357}{1} \times 100 \%=64.3 \%\]
against the US Dollar.