I'm not sure it's a valid claim, but the author implies there is a significant difference in the quality of the evidence:
>In fact, scientists have already proposed that one such group of animals called loriciferans can do that, and had some evidence that this was the case, although not as much or as detailed as for Henneguya.
On the other hand, this might just be a justification for low key sensationalism.
Here's the source BBC article, from 2010: https://news.bbc.co.uk/2/hi/8609246.stm, the headline is "First oxygen-free animals found"
https://en.wikipedia.org/wiki/Spinoloricus_cinziae