The best they could come up with in that they aren't doing enough to investigate/combat this? Or that this isn't worth investigation in the first place?
No corroborating IG investigations or official reporting or even FOIA suits/events, which a serious external investigation would use.
That doesn't make it impossible, just heavily unlikely. Furthermore, it has all the hallmarks of an unsourced smear campaign then used as a convenient 'source' for sensationalist reporting. If the average HN reader were to inspect that source, I exepct they would see the lack of investigative rigour and wonder how the word "source" could even come into consideration here.
If you think the IG or direct lawsuit routes are unfeasible, there are tons of congresspersons on both sides of the aisle that would jump on that to get one over on the administration, as well. Their oversight powers do have some bite.