This is because their proof uses trigonometry.
Now why is that such a big deal? Well, many of our trigonometric identities and laws depend on the Pythagorean Theorem, and so a number of mathematicians have suggested that any proof of the theorem using trigonometry is circular logic. Put another way, they argue that using trigonometry to prove Pythagoras is basically using A to prove B, when A already depends on B. One strong proponent of this point of view was the mathematician Elisha Loomis, who published a book in 1927 full of non-trigonometric proofs of the theorem, and explicitly stating that trigonometric proofs were impossible.
However, this point of view has been increasingly questioned in recent decades, and a few trigonometric proofs of Pythagoras have made the rounds since then. Claims in the media that Johnson and Jackson’s proof is the first trigonometric proof of Pythagoras are overblown, but their proof could well be the most beautiful and simplest trigonometric proof we have seen to date, and is clearly the work of young, sharp minds uncomplicated by the years of deep research that characterise the work of many experienced mathematicians.