إِذْ لَمْ يُسْمَعْ كَلِمَةٌ بَلِيغَةٌ
For the expression “a baligh word” is not attested in the usage under discussion.
The argument appeals to established Arabic usage. The text is defining where the technical description applies, rather than assuming that all semantically related words can receive exactly the same label.
This does not imply that an individual word lacks meaning or expressive force, nor does it establish an absolute claim about everything ever said. It states the scope of the classical technical attribution: a fasih word, but balagha of discourse and speaker.