Coinbase's Lend product pools USDC from customers, and lends them out to ??? who does ??? and pays Coinbase interest for it. Presumably the ???'s are "crypto traders" and "trades crypto, possibly with leverage", respectively. Those specific details dont matter, though.
According to the SEC: "To determine whether a transaction involves an offering of 'securities,' courts employ a four-part test outlined by the Supreme Court’s 1946 decision in SEC v. W.J. Howey Co. Under that test, a transaction qualifies as an offering of 'securities' if it involves:
(1) an investment of money,
(2) in a common enterprise,
(3) with a reasonable expectation of profit,
(4) to be derived from the efforts of others" [0]
I'm not a lawyer or a securities regulator, but it seems hard to see why Coinbase Lend doesn't qualify as a security under those rules.
[0] https://sgp.fas.org/crs/misc/R45301.pdf (coincidentially this PDF is about ICOs, but isnt directly related to the current SEC action against Coinbase).