But there will never ever be such guarantee that could be taken seriously. How do you know that 2 star generations ago (so say roughly 10 billion years ago), in our previous neighborhood, a massive hypernova wiped out advanced civilization and you are now just consuming molecules that were once part of sentient living being, even with IQ much higher than yours?

Nah, you can never be 100% sure, its one of those situations reachable only on paper.