What do you mean? ... that this was a "retroactive prediction"? This is what I suppose you mean, but I don't see what's wrong with the headline on this basis.
It’s not an experiment. It’s simply the best detail calculation of the predicted masses of protons and neutrons based on QCD done so far. The headline is perfectly accurate.
As it happens, there have been literally thousands of other measurements of proton/neutron masses and each of those has been a confirmation of the standard model. That by itself is utterly trivial.
If you consult the article, the entire point is that they have confirmed an aspect of quantumchromodynamics. So you'd think that's what the headline would be about.