I would want to see some kind of statement from him to this effect to be sure of it, but if so, this is groundbreaking.
It seems like Isaac may be open to correction of his formulation.
Avi proposed this in the Qwerky discord:
While it looks like it still requires the Identity of Indiscernibles premise, aside from that it seems to cover all of the major semantic contentions mentioned in the wiki.this isn't fully ironed out so this may have sementic issues too
but the general idea can work
P1) An observer convinced that he is observing all humans must conclude that at least one human he is convinced of observing has moral value
P2) There is no trait (accidental or essential) present in animals in which if realized to be present in what the observer was convinced was a human with moral value would result in the observer concluding that the resultant of the now realized trait application would not have moral value
C) An observer convinced that he is observing animals must conclude that what he is convinced of observing has moral value.
By framing P2 in such a way, it allows for essential traits differences to be analyzed without any internal contradictions. There is no internal contradiction with an observer thinking something was a human and then realizing he made a mistake about an essential property and then realizing that the resultant was not a human and never has been a human. The question then becomes: after the observer realizes he was wrong about the essential trait, does the observer conclude that the resultant (what the observer realizes the thing he thought he observed actually is) have moral value? P2 states that the answer is yes. Thus, if one accepts P1 and P2, C must follow.
So basically, an observer observes a person who is 6 foot tall (accidential property), then realizes he made a mistake and the person is 5 foot tall. Does the observer change moral valuation to the person? If yes, that's a rejection of P2, is no, that's not a trait.
the same can be said for essential properties
I haven't spent much time with it, but it appears that a grandfather clause would prove its invalidity; that is, once you consider something to have moral value, you can't change your mind based on correction of perception or actual change of traits.
So if you mistook a cow for a human and considered the being to have moral value based on whatever necessity drove that consideration, you can't change your mind once you realize it's a cow. But if you knew it to be a cow originally you could claim it valueless. This exploits the temporal asymmetry in the second premise and conclusion without having anything to do with a trait (just the moral system itself).
I could be wrong, though. As I said, this is just from looking it over briefly.
If this flaw is true, I'm not sure if this would be easy to fix or not. I'll work on a fix later, just hoping to get feedback in the meantime.