That omission does not render an agreement invalid. If anything (and that's shaky because the document is primarily a separation agreement, not primarily a waiver of rights), it would invalidate only the portion pertaining to waiving ADEA claims. The law has long provided that errors in omission do not render entire agreements or contracts invalid.
There was a case I had to read in college where some whack job found a legitimate omission in Microsoft's EULA for Windows--that he said invalidated the entire EULA--and therefore he (the claimant) said he was legally allowed to copy and sell pirated copies of windows. There are tons of other cases out there.
If you're suggesting that CD would use the omission as a loophole to sue, he'd have to prove that not being told by the U that he had 7 days to renege on the waiver was more harmful to him than the severance package he got. Good luck.