If there’s an existing structure where the moisture content (MC) is less than 19% for southern pine timbers, could the designer back out the wet service adjustment from the NDS published design values in Table 4D?

No. The published Southern Pine timber values are for wet and dry use. The Southern Pine grade rules, section 164.4 state:

“Where lumber is over 4″ thick, the stress ratings apply without regard to seasoning, and stress ratings as herein indicated for a moisture content over 19% apply also to seasoned lumber if over 4″ thick.”