The book gives “Zayd” and “ʿAmr” as examples of particular concepts. “Zayd” here means an already identified individual, not merely the sequence of letters in the name.
“Zayd”: one particular person.
“ʿAmr”: another particular person.
“Human”: a nature that can be shared by many individuals.
Two people can both be named Zayd. This does not make them numerically one individual. A shared written or spoken name is different from the specific identity intended whenever the name is used.