The distinction between noun and verb categories varies across the world's languages, with some lacking strict or robust grammatical differentiation, making citations unnecessary for this widely recognized linguistic observation.
The claim addresses a well-known typological fact about natural human languages—that grammatical categories and word classes are not uniformly partitioned with sharp, universal boundaries across all tongues (some languages, for instance, exhibit fluid word classes or flexible root systems where words can function as either nouns or verbs without distinct morphological marking). This is a matter of linguistic typology and everyday descriptive observation in linguistics, rendering empirical citation unnecessary. Therefore, the verdict is COMMON_KNOWLEDGE.