However, he implied a false fork that, if moral realism is false, then humancentrism is the answer.

I don't think he would put it that way. He defines good as “that which leads to sentient beings living, to people being happy, to individuals having the freedom to control their own lives, to minds exploring new territory instead of falling into infinite loops, to the universe having a richness and complexity to it that goes beyond pebble heaps, etc.”, not as ‘what humans value’, and considers it a “moral miracle” that humans value what leads to sentient beings living etc. etc. (Of course, the reason why we're talking about what leads to sentient beings living etc. etc. in the first place is that that's what we value, so IMO being surprised that we value that would be --as Feynman put it (though he was talking about something else)-- like being surprised that I can see the car with the number plate AC 443 MW.)