A theorem in mathematics is a proven fact. A theorem about right triangles must be true for every right triangle; there can be no exceptions. Just showing that an idea works in several cases is not enough to make an idea into a theorem.
The Pythagorean theorem has been proven to work for every possible right triangle. Of course, you can't draw every right triangle on graph paper, make squares on the sides, and find their areas.
Many people have written proofs of the Pythagorean theorem. In fact, whole books exist that contain nothing but proofs of this one theorem! The proof that follows is probably from China, about 200 B.C.E. Rather than learning of it from the Pythagoreans, though, the author of the proof most likely developed the theorem independently.