Is Singer even serious?

I see no reason to doubt that he means exactly what he says.

I mean, how isn't it OBVIOUS that would not be an argument against eating animals but an argument for eating humans?

Modus ponens, or modus tollens? White and gold, or blue and black?

Because unethical behavior is the default and ethical is the special case. Take away speciality and it is back to the jungle.

On the whole, we observe that people naturally care for their children, including those who still live in jungles. There is an obvious evolutionary argument that this is not because this has been drummed into them by ethical preaching without which their natural inclination would be to eat them.

To me it is so obvious I hardly even think it needs much discussion...

To be a little Chestertonian, the obvious needs discussion precisely because it is obvious. Also a theme of Socrates. Some things are justifiably obvious: one can clearly see the reasons for a thing being true. For others, "obvious" just means "I'm not even aware I believe this." As Eliezer put it:

The way a belief feels from inside, is that you seem to be looking straight at reality. When it actually seems that you're looking at a belief, as such, you are really experiencing a belief about belief.