That's a pretty clickbaity paper title. The lede was buried. A better title would have been: Regularly replacing HEPA filters may reduce endotoxins in filtered air
I'm a high school drop out so what do I know.
Bit strange given that you've removed the alleged source of the problem (stuff caught in old filter).
e.g. A car with a catalytic converter avoids increases of certain gases in the air, but it doesn't reduce what is already present.
In that light both the title and the suggested mitigation seem pretty straightforward.
What it's not clear to me is the definition of "purifier" and they operating principle: VMCs are normally harmless because they do not blow humid air and they keep air flowing so it's hard to offer room for "colonization" in most cases. That's the same principle we have for modern water heaters, classic ones, large pressurized balloons filled of hot sanitary water do require a regular daily extra-heat-up against legionella, modern ones with a not pressurized hot-water balloons and just a spiral pipe inside to "instantaneously" heat the sanitary water flowing in the pipe in most country do not demand anti-legionella heating cycle because they do not offer room for it's development. So much of the point of this study should be define the kind/family of devices they have testes.