That relies on the “tired light” hypothesis being correct. It solves a number of problems, in a more elegant way. However, it also requires explanations for some new mismatches. E.g. why the cosmic background radiation doesn’t seem to have aged the same way.

It’s a theory that can’t be immediately dismissed, which makes it interesting, but it’s far from proven. Scientists can now look for details that would differ between the 2 models, and so help clarify what is happening.