A few points

I also eat plenty of insoluble dietary fibre.

Prey animals would need to signal their non digestability to their predators.

Animals without predators would overwhelm the biome.

Disease is a predator.

I just ate some bread

Sounds digestible and non-sequitur.

I believe what they're getting at is that it's possible to do things to other species which leaves them digestible when they weren't initially. For example, human cooking, bird gizzards, cows chewing their cud, or rabbits eating their feces to give a second pass of digestion.

You talk about cooking meat, but we still have enzymes in our stomachs (e.g. pepsinogen) that only digest L-peptide chains. As a predator, you do not digest D-peptide chains.

- It is analagous to eating raw grass.

- It is not analgous to eating processed/cooked grains, hence it is non-sequitur.

- It is a matter of biochemistry, not culinary arts.

I gave the original speculation as an example of there being too much speculation around abiogenesis and evolution. This discussion proves it.