This is an important point that you need to understand, the Fannie Mae and Freddie Mac guarantee of the timely repayment of principal and interest to investors in their MBS ARE NOT GUARANTEED BY THE FULL FAITH AND CREDIT OF THE UNITED STATES GOVERNMENT.
EVERY SINGLE MBS issued has that statement conspicuously stated in BOLD on the front page of the Prospectus. Thus, the federal government is implicit NOT explicit.
Everyone in the financial world understands that given the $7.5T in outstanding GSE MBS and their importance to the US housing market and world economy that the US Government will come in and rescue GSE MBS Investors.
Since the FHFA and UST have Nationalized the GSES and have 100% control over them, including all their Economic Rights, the US federal government owns the GSES and therefore the guarantee of timely repayment of principal and interest to all GSE MBS Investors. The premise that Fannie Mae and Freddie Mac are private corporations has been destroyed by the actions of the federal government during the 14+ years of CONservatorship.