The really awkward thing is that human level isn't a discrete level but a wide range based upon the set of all humanity. Not just our assumptions of such. If a new outlier was somehow born with the intelligence of a mealworm or what was previously believed to be a level only reachable by a superintelligence those vast anomalies would still have human level intelligence.
We don't actually apply the set based standards properly but instead based upon imaginary ideas of human level. If we did consistently operate on set logic and the directive people reduced to brainstem autonomous functions only would eithervmake eating plants unethical or we would be arbitrarily excluding humans from the set for reasons other than absolute ethical.
To go more fantastic it would be consistent with ethics to declare obligate murderous vampires who must eat humans non-human because their existence literally depends unconditionally on taking human lives and it falls into paradox of tolerance.
Instead our standard for the irreversible brain stem only is to eventually withdraw life support or harvest their organs as they are effectively judged dead.