You could reply to my comment instead of obliquely referencing it here.
>this very clear and explicit definition
This is a definition of an exchange (specifically a securities exchange), but nobody is contesting that Coinbase has an exchange product, just whether the assets for which they operate the exchange are securities or not.
Coinbase Earn is not "bringing together purchasers and sellers", because people staking their crypto are doing neither. Putting up some collateral in exchange for the privilege to validate blocks and collect a reward for doing so cannot possibly fall under a reasonable function "commonly performed by a stock exchange".