No, the CDC did not admit anything. Please, read the article, source material, and compare the claims.
The CDC released scenarios for "planning purposes only", and parameters in those scenarios are estimates and "not predictions of the expected effects". What's the difference between estimates and predictions? No idea, but this is what the CDC actually said.
As to the numbers, their current best case scenario lists 0.004 as the Symptomatic Case Fatality Ratio. The same scenario lists 35% (not 50% as the source tweet highlights) as percent of infections that are asymptomatic. Therefore, the symptomatic infections account for 65% of the total.
Therefore, as CFR = DEATHS/CASES, true CFR will be DEATHS/(SYMPT.CASES/0.65) or
DEATHS/SYMPT.CASES * 0.65 or
0.004 * 0.65 or
0.0026
So it appears, it is a case of "using the wrong formula but getting the right result".
And the CDC never did these calculations or made any conclusions whatsoever.