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 facilitate composability without blockchains if data access is unrestricted. Blockchains provide permissionless composability, but are not always essential for end products