comment on the significance of 'the thing which is not' in context to the houyhnhnms concept of truth?

In the Houyhnhnm's concept of truth "the thing which is not" is a lie.

"He replied, “that I must needs be mistaken, or that I said the thing which was

not;” for they have no word in their language to express lying or falsehood. “He knew it was impossible that there could be a country beyond the sea, or that a

parcel of brutes could move a wooden vessel whither they pleased upon water. He was sure no Houyhnhnm alive could make such a vessel, nor would trust Yahoos to manage it.”