As I understand PoH, blocks are instead continually proposed, but block producers are rate-limited by having to show completion of an operation similar to a verifiable delay function. Therefore, block producers avoid having to all compete for the same slot in the blockchain and duplicate lots of work. This is actually a novel Sybil control mechanism that is more efficient than standard PoW/PoS. It's somewhat analogous to the difference between a dedicated communication channel (as in landline phones) and a packet-switched communication channel (as in the internet).
The catch is that the PoH operation approximates a verifiable delay function, but is not currently proven to be equivalent to one. So there's the possibility of a black swan event where someone discovers a clever way to speed up the PoH operation, allowing them to cheaply control the network. Another knock against Solana is that although it has innovated in transaction efficiency, its token distribution/crypto-economics may be less "fair" than competitor blockchains. Please correct me if you spot any mistakes.