The diamond’s carbon-dated ‘age’ of <58,000 years is thus an upper limit for the age of the whole earth.  And this age is brought down still further now that the helium diffusion results have so strongly affirmed dramatic past acceleration of radioactive decay.
It is thought that the biomass before the flood was 100 times or more what it is today.  This would further dilute carbon 14 by a factor of 100, which is equivalent to 6 or 7 half lives.  This would increase carbon 14 ages by about 40,000 years.  Also, there may have been less carbon 14 in the atmosphere before the Flood.