Completely untrue. Gene silencing by the RNAi pathway would have no effect on the vast majority of mutations. The pathway is effective against exogenous double-stranded RNA, such as is produced in the life cycles of some viruses, and perhaps against the self-complementary inverted repeat sequences found in the transcripts of retroviruses and retroposons.
You say that it is not true that it prevents some mutations and yet you give examples of some mutations which it prevents! So regardless of the semantics of the matter it is part of the array of methods which an organism has to prevent mutations. It should be noted also that both viruses and cancers often work by inducing hyper-replication of cells and this directly fights this problem.