As I understand it, there doesn’t need to be a liquid market for something in order for it to be a security; just an investment of capital with the expectation of return through the efforts of a third party (the Howey test)
According to Coinbase, the SEC said "they consider Lend to involve a security." Not that it is a security. The second question is (a) more ambiguous and (b) a red herring from Coinbase.
If Coinbase is taking deposits from customers to engage in securities trading, it's acting as a bank or broker-dealer. It is registered and regulated as neither. Note that Kraken and Gemini have state banking charters. Coinbase is drawing a false equivalence. The entire enterprise stinks of bad faith, possibly fraud.