Generational collectors need to record when older generation objects are modified (by card marking, for example), so they can distinguish mutated from unmutated.

Afaik card marking and other similar schemes do not care about (or track) mutated objects. They track cross-generational references, which have to have been caused by mutation but only a very selective subset thereof. And card marking does not even track at the object level, it tracks pages which have at some point contained a pointer to the newgen.