They do guarantee some loans, as I wrote above. The marketing blurb you copies cleverly does not state they will guarantee any mortgage loan.
If they did, the housing crisis would not have been such a crisis - the crisis happened when so many investors realized the loans they bought could crash and cause them massive losses - precisely because FM does NOT guarantee all loans.
They guarantee certain loans (conforming loans), and charge a fee to investors for this when they purchase MBS from Fannie Mae. Conforming loans make up under 40% of all mortgage loans. Of all conforming loans made, Freddie and Fannie end up purchasing about 60% of those.
It may be that they stopped buying loans they deem too risky, but that didn't use to be the case.