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

Open APIs can enable composability/interoperability without needing blockchains. Blockchains offer permissionless composability, but open data can achieve similar results without blockchain