Whether it is a deduction or a credit depends on how large the adjustment is. If the adjustment is $3,000 or less you have to take it as a misc. deduction which must be over 2 percent of your adjusted gross income. As you suggest, this won't help much.
However, if the adjustment is more than $3,000 you get to figure out how much additional tax you paid due to the taxation of SS benefits in any year back to 1983, find the sum of that additional tax you paid, and then subtract it directly from your total tax bill for the current year. I read that as a direct tax credit for prior taxes paid on SS benefits.
As a practical matter, no one will be going back to 1983. Since your SS benefits stop growing at age 70, no one older than 71 or so is likely to reapply and they'd only be able to go back 8 years to age 62 if they started taking benefits at 62. So a 70 year old who reapplied in 2008 would only have to go back to the year 2000 while someone who started at 65 would only have to go back to 2003.
Scott