It may be true. African slaves were chosen from select people known for their longevity, resistance to disease, tolerance to harsh environments. great stamina and a history of timidity.

From a book called "The Panorama of Life and Literature", Human Longevity, Vol. V, July to December 1857.

1840 United States Census.

Over a hundred years in age.

Whites... 791
Slaves... 1,333
Free Colored... 647.

That's well over twice the total number of Whites.

1855 census...

Oldest white male was 110.
Oldest white female was 109.
Oldest Black female was 120. (a slave)
Oldest Black male was 130. (a slave)

Taking the census from 1790 to 1840 they found the chances of living over a hundred years of age for slaves was 13 times, and for freed blacks 40 times as great as in the white population.