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 are sufficient for composability/interoperability, but blockchains offer permissionless composability. Non-blockchain tech with open data could also achieve the same results without the need for blockchains