Spot settled means it should be physical bitcoin in Coinbases custody. Although last i checked we have no way to audit what they hold. There's definitely a transparency issue with a majority of the ETF's.
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Ironically, that’s the appeal of BTC. Transparency issues.
coinbase can change a number on their ledger if that's the standard.
one of the ETFs posts their addresses. If they would move off of coinbase and post their addresses I bet the price would move with ETF purchases more.