Here's the quote from the abstract that I found revealing:

Finally, recent clinical trials with doxycycline and azithromycin (intended to target cancer-associated infections, but not cancer cells) have already shown positive therapeutic effects in cancer patients, although their ability to eradicate cancer stem cells was not yet appreciated.

So, the way I read it, "It's a good idea, but we haven't gotten it to work yet, even after some testing, but it's a good idea."

I've been around a few research science labs and understand the pressures on them. Getting published is good...but not sure of the provenance of this outlet.

Unfortunately, a lot of negative research results aren't published, so it may be the case that follow-up studies were unsuccessful.