It is. There's evidence of its use in the Old Babylonian period, evidence in 1800 B.C.E Egypt, India in 700-500 BCE, China during the Han Dynesty at least.

It's very simple to prove, and anywhere you find squares or triangles in architecture, it was used.

People can re-invent and re-discover things. It still happens all the time in this day and age of worldwide massive communications. I'd be surprised if the right angle theorem didn't get discovered thousands of times throughout history.

Everyone learns something new everyday. How often have you seen a TIL and thought, "doesn't everyone know that"

Browsing the wikis, I got the impression research is unconclusive. We don't know if he had a role regarding the theorem, and what it was.

There is debate whether the Pythagorean theorem was discovered once, or many times in many places, and the date of first discovery is uncertain, as is the date of the first proof. Historians of Mesopotamian mathematics have concluded that the Pythagorean rule was in widespread use during the Old Babylonian period (20th to 16th centuries BC), over a thousand years before Pythagoras was born.[68][69][70][71]

The German version also talks about the various roles Pythagoras might have had or not had regarding the theorem, and how research is unconclusive. One such possibility is that this older Clay Tablet applied the theorem without being able to prove it, and Pythagoras or one of his students could have found a proof.

Also:

The history of the theorem can be divided into four parts: knowledge of Pythagorean triples, knowledge of the relationship among the sides of a right triangle, knowledge of the relationships among adjacent angles, and proofs of the theorem within some deductive system.

So there were lots of meaningful steps one could achieve without actually deriving the theorem. Maybe people were happy to just use math because it works, and a thousand years later someone bothered to prove why.