I don't like that phrase. It suggests the possibility that this galaxy might not exist, that this could be observational error. I think it does exist. We understand exactly how it exists because we can observe it existing. The more accurate phrasing might be "it's not understood how it could have evolved so quickly".
This could absolutely be observation error. Messups in science happen all the time. You never take a single data point at face value without question, in any field. The spectrum might be polluted by another more distant object, the line matching might have been faked by some other intervening medium, the modelling for the lensing galaxy could be wrong. I agree, it looks good.
But it could totally be a mistake.
Remember not long ago when neutrinos were thought to have been observed moving faster than the speed of light? Cold fusion?
Before people jump on the idea of re-evaluating Einstein's theories, it's always good to get more people looking at the data, independently verifying it, thinking about it. In a few months, it's likely that this blip will disappear like most others.
>the theoretical models are wrong, or at least (and more likely) incomplete.