It's true that Ruby is a Japanese invention (I wonder if all these racists are aware of what they're bringing into their good ol' boys' club ;)), but as you note, Japan is so far away that there are still difficulties in participating in a little new conference in Manchester. Also, Ruby got its big break with Rails, which was developed in Chicago, I think, so the Japanese community might not have gotten as much of a headstart as one would expect. Or maybe they are using Ruby for far more interesting things than web development. I don't know.
And I think that's the heart of the matter...The Japanese (or any number of non-European/American Ruby communities) could be doing something amazing...and we would have a hard time knowing, because of the inherent geographical and language barriers. That's why it's not just the "moral" thing to make an effort toward diversity, but it is a potential creative and intellectual boon.
In any case, the OP should've just included a statement saying, "we asked speakers x,y,z, for example, but each had a previous commitment, etc. etc. etc.". In fact, that's a point they should've made more prominently in the debate before canceling the conference.
I agree with #2, but Ruby is a fairly special case, in that its creator, and still one of its main contributors, is Japanese. So you can't make the argument "if only non-whites stepped it up in Ruby dev"...(but the argument that none of the Japanese speakers could make it/were affordable/wanted to come is still legit).