"Neither true nor false”
means that the statement has no definite truth valued : it lives in a sort of limbo, a truth value-gap between true and false.
“Either true or false”
means that the statement has (exactly) one of the two truth values.
To say that "Lexical definitions are either true or false" means that a Lexical definition :
also known as dictionary definition, is the meaning of the term in common usage. [..] Note that a lexical definition is descriptive.
Thus, it states a fact : that the defined term has a certain meaning, and it cab be right (true) or wrong (flase).
A Stipulative definition instead, is a sort of convention :
in which a new [...] term is given a new specific meaning for the purposes of argument or discussion in a given context.
Thus, it has no sense to ask if it is true or false.
Compare OR with NOR.