"Nec si miserum Fortuna Sinonem "
Nec si miserum Fortuna Sinonem
Finxit, vanum etiam, mendacemque improba finget
"Though Fortune had made Sinon wretched, she has not made him untrue and a liar"
The above quote is from Vergil's Aeneid. The incident takes place at the end of the Trojan War when the Greeks have built a large horse and filled it with warriors. Sinon lets himself be taken prisoner and tells a sad, false tale of having been mistreated by the Greeks, and states that the wooden horse is an offering to Athena and will forever protect Troy if it is brought within the city. The Trojans bring it in, the warriors emerge at night and Troy falls. Sinon is thus the archetypical traitor and he is consummately "untrue and a liar."
Swift knows that and it is his way of satirizing the truth of travel tales, even Gulliver's.