Depends. If the breadcrumbs are key material which correlates to other known incidents from the same group, or exclusive tooling, or C2 infrastructure, then there is definitely something stopping them from putting breadcrumbs there. They'd have to hack the other group first in order to do so.

I agree with you that seeing evidence would be nice, but I understand that there is the possibility that evidence supporting the claim exists and at the same time cannot be released to the public.