In principle, diverse double-compiling merely increases the number of compilers the adversary needs to subvert. There are obvious practical concerns, of course, but frankly this raises the bar less than maintaining the backdoor across future versions of the same compiler did in the first place, since at least backdooring multiple contemporary compilers doesn't rely on guessing, well ahead of time, what change future people are going to make.

Critically, it shouldn't be taken as a demonstration that the toolchain is trustworthy unless you trust whoever's picking the compilers! This kind of ruins approaches based on having any particular outside organization certify certain compilers as "trusted".