I disagree. Avoiding PKI and post-quantum security correlate very much. Even under plausibly post-quantum assumptions we only have a couple of assumptions from which we can build public key encryption. In contrast, here they avoid all use of public key cryptography which makes it provably post-quantum secure. It’s not using a buzzword for the sole sake of selling the paper. In general, using “minimal cryptography” (like random oracles / one-way functions) translates to real-world efficiency because you can instantiate these from a plethora of different concrete candidates.