It'd be good if the Guardian stopped being so dumb:
This is about a proof that is supposedly "solely trigonometrical". But basic knowledge of trigonometry will tell you that its formulae (eg for relations of sin, cos, tan etc) are dependent on the pythagorean theorem being true (=> you are using formulae that are true due to x, to try to prove that x is true).
Good for the highschool kids trying something cool, but it is well established that trigonometric formulae being true implies pyth theorem is true.
This is an abstract from the kids' proof:
But the
law of sines is true (though implicitly) due to the inferred right-angle connections between those non-right angles.
Trivia: you can also read the law of cosines, in Euclid's book II (propositions 12-13), again as direct consequence of pyth.
Re the Guardian, seriously, Pythagoras lived in the 6th century BC. Do the elementary math about the theorem's age, it is 2500 years old

^^
All
that said, while their proof likely won't be accepted as being "solely trigonometrical" (since law of sines is still tied), it is a very elegant new proof, and worth it for that alone
Their actual construction seems to be this, by the way:
It's not ingenious or wondrous, but it is elegant. My suspicion is that the mere fact that properties of right angles are used in the definition of what sin is (because it has to be in a right-angled triangle to start with, then extrapolate to the related non-right one), already means the ratios between angles imply the pyth theorem. But obviously this proof can be part of books of proofs of the theorem, despite not being "the fully trigonometric one" as marketed.
Sidenote: if they had marketed it as such on purpose, that was
really a great idea
