This doesn't seem like proof to me.
The author got a phishing call and reported it. Coinbase likely has a deluge of phishing complaints, as criminals know their customers are vulnerable and target their customers regularly. The caller knowing account details is likely not unique in those complaints; customers accidentally leak those all the time. Some of the details the attacker knew could have been sourced from other data breaches. At the time of complaint, the company probably interpreted the report as yet another customer handling their own data poorly.
Phishing is so pervasive that I wouldn't be surprised if the author was hit by a different attack.
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