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.)

Our amazement.— It is a profound and fundamental good fortune that scientific discoveries stand up under examination and furnish the basis, again and again, for further discoveries. After all, this could be otherwise. Indeed, we are so convinced of the uncertainty and fantasies of our judgments and of the eternal change of all human laws and concepts that we are really amazed how well the results of science stand up.

• Nietzsche, The Gay Science I.46

(NB: in this passage, "we" signifies modern atheists, not people in general.)