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> The scientists from RIKEN and their collaborators took up the task of developing transportable optical lattice clocks that could make comparably precise tests of relativity, but on the ground. The ultimate purpose, however, is not to prove or disprove Einstein.

What they are testing is their ultraprecise clocks, not the theory of general relativity like the headline says.

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Sure, but all of these things are always interlinked. It's entirely possible that the first deviation we ever find from general relativity will come in the form of just trying to make better and better clocks, but then finding some funny systematic effect that prevents different atomic clocks from properly syncing up, and then ruling out all other explanations. This kind of discovery is not without precedent in physics.

The clocks were of course compared in previous experiments with no height difference between them.

The Skytree experiment is directly measuring gravitational time dilation which is a central prediction of General Relativity. Combined with an accurate measurement of the height difference and local gravitational acceleration, this makes it a very accurate test of GR.

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