I was thinking, you don't really need blockchains for composability/interoperability, you just need open APIs

permissionless composability, blockchains do provide

but if there were theoretically some non-blockchain tech that just exposed open data, and the end-result product that takes advantage of this data is the same, then the blockchain part would not be necessary for the end product

If you just need access to data, yes, this is IPFS. But if you need to trust the data source, the action that was taken, etc, then you need a source of trust. The only decentralized source of trust tech I know is blockchains.