I purchased and withdrew 1 bitcoin today from Coinbase. The entire transaction took less than a minute from purchase to withdrawing the bitcoin to an external address.
Ok, but OP sold a coin so that was fiat going from Coinbase to the OP's bank. Yours was btc going from Coinbase to your external wallet. Different things.
No, they are not different; when you purchase the allowed 1BTC after the verification steps, it is guaranteed by the credit card on file until it clears your bank account, and with the speed of ACH these days it is almost an instant transaction with your bank behind the scenes. The delay in it appearing in your bank account as an ACH withdrawal is a security step done by both banks to ensure that the bank accounts are real and funds are actually present and not the result of you defrauding your bank with a bad check or a dubious money order scam.
When the bank account was verified by each member of the site, it was a statement of mutual trust between you and Coinbase (They can deposit and withdrawal any amount present at any time -- A good reason to only associate an account that you keep nothing in.)