how did the interpreter at Luggnag correct Gulliver's misconception about the immortal Struldbrugs ?

The Struldbrugs are said to be immortal, but for the most part Gulliver finds that they begin to "decline" after the age of thirty and usually are dead by the age of eighty. If they do not die, they are sometimes eliminated, so the myth of their immortality with all of the hype of wealth and riches is simply that: a myth.