The Pythagoreans would say just the opposite: That Plato took Pythagorean notions of a mathematical universe and twisted them out of bounds.
Plato's notion of the abstract "intelligible world" or "world of forms" is especially a violation. Consider the writings of the Neopythagorean Plotinus, in a treatise titled "On the Question of Whether There are Ideas of Individuals":
"If, though, the soul of each contains the expressed principles of all those it will successively enter, then, again, all will be in the intelligible world. And we do say that such expressed principles as the cosmos contains, each soul also contains. If, then, the cosmos contains the expressed principles not only of human beings but also of individual animals, so, too, does the soul. There will be, then, an unlimited number of expressed principles, unless the cosmos is recycled periodically, in which case the unlimitedness will reach a limit when the identical things recur."
Here Plotinus quite correctly notes that eventually all things will wind up in the abstract intelligible world, each with their own form, until they recur. For there can be no unlimited number of expressed principles, the number of combinations of matter in space and time being finite, and in the end everything is its own irreducible form of ideal.
In not understanding this, Plato arguably didn't understand Pythagoras at all.
And of course Pythagorean sayings like "all is number," "things are the same again in number," etc. suggest an entirely classical understanding of a mathematical universe hypothesis. What's more, the Pythagorean abhorrence of irrational numbers hints at the distinction that Tegmark and others later draw: That they had two classes of mathematics -- a "realist" mathematics, and an "irrational" mathematics.
> (It's Nietzsche who points out that the "eternal return of the same" is the only basis for an individual ethics that doesn't resort to a greater surrounding context.)
The Eternal Return is another Ancient/Pythagorean concept that has been hotly debated over the past 2600 years. Short of popularizing the concept, Nietzsche has not contributed much to the debate, and his words on the topic are far from definitive.