Of course it's possible to try all possible keys, and by definition that's parallelizable, but the idea is that that should be hard enough to be infeasible. Let me give an example that should be impossible according to your reasoning: Imagine I (magically) had a method that allowed one to disclose information that would allow someone to figure out an RSA private key after two years. That would clearly satisfy gwern's original requirements - it's easy to encrypt my data, publish the public key and the "magic hint". Yes, an attacker could just try all possible RSA private keys in parallel - but that's not a realistic attack.