The administration only has authority to make and enforce rules about a law to the extent that Congress delegated that authority to them. Sometimes Congress does so. E.g., the FCC was created by Congress in the Telecommunications Act of 1934, with the authority to make rules regarding various things specified in that law.

If Congress does not so delegate such authority, interpreting the law is solely the job of the courts. They did not delegate such authority for section 230.

Agencies do not get to just look at any random law that is outside the are they were given power over, decide it is not clear, and make an enforceable rule clarifying it.