The Pythagoreans, broadly speaking, were interested in the way in which geometrical mathematical proofs told deep, transcendental truths that seemed to get beyond the mundanity of the everyday. They generalized this outward as a model of truth and a true nature that hid beneath the messiness of the apparent reality, but to my knowledge they did not engage in either quantification (assigning numbers to everything) or reductionism (trying to simplify everything to simple causes or formulae). They were, in a modern categorization, people who existed at the boundary of mathematics and numerology (number worship).

Later thinkers took their model of truth and attempted to extend it and even move it into a more firmly empirical realm. Plato challenged his followers to uphold the sort of perfection that the Pythagoreans saw in math and apply it to nature, in particular to places (like the retrogression of Mars) where nature seemed to contradict a simpler answer.

It is far later that people sought to find simple algebraic laws to explain natural phenomena, and later still that they really began to engage in the systemic quantification of nature.

So, from what I know of the Pythagoreans (and I am not an expert on them, but I have taught on the general trends above), I would say, no, that isn't really what they believed. If you wanted to sum up the stereotypical Pythagorean belief, you'd say they believed that numbers and mathematics revealed deep, quasi-religious (or just straight-up religious) truths about the world. That is not the same thing as saying that the world can be explained in numbers or reduced to them.