hong on Nostr: trying to understand the chainlink proof of reserves product for bitcoin etfs. for ...

trying to understand the chainlink proof of reserves product for bitcoin etfs. for example this is the site for arkb (https://data.chain.link/feeds/ethereum/mainnet/arkb-por)

how does this work? seems like it's not really tied to the bitcoin blockchain and is simply a custodian attestation but how do the chainlink oracles get access to arkb's bitcoin balance at coinbase? it's non public information no?