This word in this context means ‘distinction’, not ‘limit’ or ‘boundary’ as all the dictionaries register.
Πρώταρχος
πῇ ποτε διορισάμενος οὖν ἄλλην, τὴν δὲ ἄλλην θείη τις ἂν ἀριθμητικήν;
Σωκράτης
οὐ σμικρὸς ὅρος, ὦ Πρώταρχε
He reminds me of the English idiom: “it’s not a fine line ”.
something like that