«إِنَّمَا زَيْدٌ كَاتِبٌ»
Zayd is a writer—and, in the relevant contrast, it is writing that is asserted of him.
Innamā (إنما) is a recognized marker of restriction. It presents the assertion while excluding a competing interpretation or attribution. Unlike “mā ... illā,” it does not place a separate overt negation and exception around the remaining predicate, yet it can convey similar confinement.
Innamā should not always be translated as an unlimited metaphysical “nothing whatsoever except.” The scope follows the context, what the listener already knows, and which alternative the speaker seeks to exclude.