Context:
From his carriage Kongming laughed. "I would have expected loftier argument from a venerable minister of the Han court, not these debased words.
It would seem to me to be a matter of style. One can use the noun 'argument' in such a manner as to encompass the whole discussion. In that case, this sentence is saying that the minister should have been capable of more erudite conversation. However, in general, I would agree that 'argument' should have been pluralized. In that case there would be no confusion.
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