The Hedera hashgraph platform is post-quantum resistant for hashing and encryption, but not for signatures and key agreement.
The hashgraph consensus algorithm itself is completely post-quantum resistant, as long as you use a post-quantum signature. We've designed our system to make it trivial to plug in a post-quantum signature scheme as soon as the current NIST contest selects one, or more.
In terms of relative security, the US government doesn’t yet require post-quantum ecnryption for protecting its own top secret information, and are willing to wait a few years for NIST to finish. Hedera Hashgraph is following the same procedure.
We will be watching the progress on quantum computation very closely. Right now, it appears we are years away from quantum computers becoming a real threat; if something happens to change, we will adapt incredibly quickly.