I think his point is the following: What if someone has been secretly building their own blockchain in parallel for the last N months. When they publish the chain some of the coins in your wallet right now would disappear, since according to the new longer (hence better) blockchain they were never mined at all.
Regular checkpoints in the reference client help avoid this being a real problem however. For instance version 0.8rc1 of the reference client has block 216116's hash hardcoded into it. So any parallel blockchain forked earlier than that block (11th Jan 2013) will never be accepted.