He’s posted pictures of dressed up chimps before as a means of being irreverent about news stories of the day and possibly I’ve seen him do it before. The fact the baby is mixed race won’t have occurred to him
The thing is, if you start up a running joke based on posting pictures of dressed up chimps, you surely must be aware that you need to tread carefully - particularly if you are a Millwall fan: you cannot help but be aware of the potentially racist connotations. So if you're not prepared to apply your mind as to whether any of those depicted could have been of mixed race, you have chosen to accept the consequences.
In this case, Baker could have rescued the situation at least in part by abjectly apologising. I simply don't understand why he hasn't done that, instead of coming up with a blatant lie that no-one could seriously believe. That is where the whole situation is so inexplicable, and it does make me wonder whether there isn't something else going on with him.