Science

A fox of the same species was found in a much older grave in another part of Argentina nearly a decade ago. It may also have been a pet but its diet was not analysed.

As usual, it's more the article (and especially the headline) than the science. Here is the Abstract of the study.

It's much more about the specific burial and the inferences that can be reasonably drawn about South America before the introduction of dogs from the north 5k years ago. It references multiple burials with non-dog canids from across time periods in S.A., including at least one from about 4k years ago, as well as many other remains scattered in with human burials. It seems to build on existing theorizing that pre-Columbian practices might have changed more slowly than post. Then there are the statistical arguments. If you occasionally find a fox in human burials, based on the number of human burials you didn't find, you can feel pretty confident that there were more foxes buried with humans.