There's some more information here: https://www.reddit.com/r/Bitcoin/comments/1jouqt/casascius_physical_bitcoins_cracked_at_defcon/. Note the top comment and the statement that the ring didn't affect the exploit.

I tried this around that time with dielectric solvent rather than non-polar solvent and it worked okay. I set the coin in a bath of solvent instead of using a needle. The sticker had a lot of residue that would have needed to be cleaned up if I wanted to reuse it, while it looks like the non-polar solvent left the adhesive useable. I didn't have the patience to try to clean the sticker and reapply it, but I think it could have been done.

If this was happening, I think we probably would see some indication in the near decade since the exploit was publicly demonstrated. Still, good to be cautious.