The NSA attack assumes an NSA level attacker has access to the root zone's (or someone else in the chain's) private key.

The NSA can then send a key for the .COM TLD that's valid, according to your pre-loaded local resolver, and they can also send a seemingly valid record for example.com.

The CA mechanism used for TLS has the same issue. It's not just theoretical either, Gmail and other Google domains had fake certificates that passed validation issued by CNNIC, China's Internet authority, which was discovered earlier this year.