Why is the distinction between *gharad* and *ʿarad* taught here?

Narrator Criticism: Corruption, Concealed Sources and Chronological Testing

• The words are always synonymous in hadith
• It accredits all of ʿAbd al-Quddūs’s teachers
• Textual criticism is limited to grammatical case endings
• Letter or vowel changes can distort the meaning of an attributed text