For any list $L,$ if there exists $3$ consecutive numbers which are sorted or reverse sorted, then the function will return false .
Here, we assume that any $2$ consecutive elements will always be different. In the question they have not specified what happens when we compare $2$ consecutive similar numbers.
Let me consider a list $L:$ $[ X_1,X_2,X_3,\ldots,X_n], n>2$
The $2$ possible input sequences for which the function returns true will be,
$X_1>X_2<X_3>X_4<X_5\ldots$
$X_1<X_2>X_3<X_4>X_5\ldots$