But... he didn't? He left plenty of details in the text for people to pick up on race if they were reading carefully. That's why lots of people did pick up on it. He also didn't get pissy. His reply is very diplomatic considering he's responding to someone questioning him about his own book when they haven't read it very carefully. He doesn't tell them they'we wrong or that they're dumb, he simply says that this assumption might call for some self-examination (which is true) and that they might get further clarification if they read the book again, but more carefully this time (which is also true).

Gaiman hardly pulled an Anne Rice here.