Something in our diets?
That's basically it. A human being that's only eaten plants has much less devastating poops.
https://en.wikipedia.org/wiki/Apex_predator#Human_trophic_le...
And "our shit contains the condensed toxins from all of the lower rungs on the food chain." lacks any credibility unless you can provide a link I have never seen.
I’m not who you’re asking, but I can only assume that they refer to the observed phenomenon of bioaccumulation, which was factor in the unexpected harms of pesticides such as DDT on bird populations.
That said, I don’t know of any risks to wildlife or the environment from bioaccumulated toxins solely due to human waste. If I am reaching and wildly guessing, I suppose folks who eat a lot of fish might pass more mercury in their waste? From what I understood, most toxins like that are processed by the liver and don’t get flushed out.