There aren't any groups that have been isolated for 20,000 years. The only conceivable way to support a claim of such isolation would be documentary evidence, and there's not only no known evidence that the islanders themselves have indigenous written records that old, there aren't any 20K written records anywhere on Earth (that are known about and/or can be read; there is writing here and there among the cave paintings, but nothing intelligible).
The ancestors of the current group got there perhaps during the diaspora which also resulted in the colonization of Madagascar by Austronesian people. Generally the date for that settlement of Madagascar ranges from 5th c BC to 4th c AD; recently there has been claims of higher antiquity. Spotty evidence of some kind of human presence (hunting/foraging?) has been known for a while.
Most of the other Sentinelese languages are either extinct, or teetering on the brink, and the 14000 acre N Sentinel island probably doesn't support a large population. Census figures by the Indian gov't are not based on door-to-door contact because of the pathological xenophobia of the tribe. There may be fewer than 50 in the entire group.
14745.6 acres, formerly two islands, but the crust shifted up due to the 2004 Boxing Day earthquake/tsunami, adding to the size of the main island and joining the islet to it; history of very intermittent contact (Tamil Chola dynasty 11th c, Maratha Empire 17th c, British Empire 18th c to 20th).
https://en.wikipedia.org/wiki/North_Sentinel_Island