The thesis at hand predicts that we should find complex things to be intricate arrangements of simple things, acting according to mathematically simple rules. We have discovered this to be true to a staggering degree, and to the immense surprise of the intellectual tradition of Planet Earth. (I mean, when even Nietzsche acknowledges this— I'll reply later with the quote— that's saying something!)

Your (b) makes no such specific predictions, and so the likelihood ratio should now be immensely in physicalism's favor. Only a ridiculous prior could make it respectable at the moment.

As for (a), I'm talking about the general principle that the world is a mathematical object, not any particular claim of which object it is. (If I knew that, I'd go down and taunt the string theorists all evening.)