The idea of "herd immunity" is that the immunized proportion is great enough that an introduction of the disease results in few infections and the disease dies out again, thus protecting members of the "herd" who aren't immunized also. For a region to be "post-herd-immunity", outbreaks coming from unimmunized communities or from places which never had herd immunity would have to be self-sustaining in the larger once-herd-immune population. Is that the case? I don't know, the article doesn't say. I doubt it, as it appears the Texas outbreak (largest in the US) remains concentrated in the particular community it started in.
So it appears to me this is sensationalism, we are not seeing the breakdown of herd immunity as a whole but rather a very large outbreak among a subpopulation that was never herd immune. There will be (and has been) spread to the general population, but so far I don't see evidence that there will be a return of endemic measles; the US used to have over half a million cases a year with a much lower population. In the meantime, might want to avoid the outbreak areas if you have a not-yet-immunized child or some reason to believe immunization has failed.