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 and interoperability without the need for blockchains. Blockchains may offer permissionless composability, but open data from non-blockchain tech could achieve the same end result