The warning is mostly that there is an oversupply of biology majors at the BA/BS level, so the common presumption that “STEM majors have good job prospects” is not true for biology majors, so biology majors who do not get into medical school (meaning most of them, since pre-med is a weeding process that eliminates most from applying to medical school, and only about half of those who apply get admitted) should be prepared to seek jobs in other areas (e.g. those which want people with a BA/BS but not any specific major).
https://www.nature.com/news/2011/110420/full/472276a.html is an article about the situation for PhD graduates. Note that, in any field, the number of PhD graduates is far greater than the number of faculty jobs available, so job prospects depend on whether there is non-academic demand for PhD graduates in research jobs.