From the wikipedia article it seems the Howey test is the legal test if something is a security:

"an investment contract for purposes of the Securities Act means a contract, transaction or scheme whereby a person invests his money in a common enterprise and is led to expect profits solely from the efforts of the promoter or a third party, it being immaterial whether the shares in the enterprise are evidenced by formal certificates or by nominal interests in the physical assets employed in the enterprise."

So presumably the logic is, lend is a security because people are "investing" securities in a contract and expecting returns not through their own efforts.

Sec has additional guidance here: https://www.sec.gov/corpfin/framework-investment-contract-an...