My understanding of the words "If the function modifies that value, the modifications appear also within the scope of the calling function for both passing by value and by reference" is that they are an error.
Modifications made in a called function are not in scope of the calling function when passing by value.
Either you have mistyped the quoted words or they have been extracted out of whatever context made what appears to be wrong, right.
Could you please ensure you have correctly quoted your source and if there are no errors there give more of the text surrounding that statement in the source material.