This hypothesis was put forth by Paris Antonio in the paper "Hydrogen Clouds from Comets 266/P Christensen and P/2008 Y2
(Gibbs) are Candidates for the Source of the 1977 “WOW” Signal".
As you correctly suspected, he tested his theory in 2017 leading to the paper "HYDROGEN LINE OBSERVATIONS OF COMETARY SPECTRA AT 1420 MHZ". In this paper he claimed to have detected a radio signal from the comets and concluded that they were the source of the WOW-signal.
It seems like this paper has mostly been ignored by the community as it is, lets say, not very convincing.
It appeared only on arXiv and the "Washington Academy of Sciences". I am not familiar with the latter one, but judging from the paper, it has not been peer-reviewed.
I will let the following sentence of the paper speak for itself:
The data collected was saved using the spreadsheet output format option of the SpectraCyber software and
imported into Microsoft Excel as a text file. The data were then replotted and interpreted using the Chart
Wizard feature in Microsoft Excel and converted into JPEG format
There is also this response to the paper, which makes very clear why it did not get any more attention:
This is a statement regarding the claim that the “WOW!” signal was caused by hydrogen emission from an unknown comet or comets. It points out inaccuracies which are fatal to the theory that the “WOW!” signal was caused by a slow-moving comet. It also points out missing details from the author’s paper.
The staff of the OSU Radio Observatory has examined the paper by Paris regarding the “WOW!” signal (Paris, Antonio. HYDROGEN LINE OBSERVATIONS OF COMETARY SPECTRA AT 1420 MHZ) and comet 266/P Christensen.
We conclude that comet 266/P Christensen is not the source of the “WOW!” signal for a number of reasons.
Using the ephemeris at:
we see that Comet 266/P Christensen, at the time the “WOW!” signal was observed in 1977, was nowhere near the position of “WOW!”. (all positions are J2000).
“WOW!” was at: RA 19h 25m 31s or 19h 28m 22s Dec -26 deg 57’
266/P was at: RA 18h 32m 15s Dec -27deg 22’.
The declination is near but the right ascension is nearly an hour off, placing 266P in a different part of the sky when the WOW! signal was observed.
Another comet, P/2008 Y2 was closer, at: RA 18h 39m 39s. Dec -29 deg 38’
The declination was almost 3 degrees off and the right ascension 47 minutes off, placing P/2008 Y2 in a different part of the sky when the WOW! signal was observed.
It is important to point out that the OSU telescope beamwidth in right ascension was only about 3 minutes (of time), but the comets were 47 and 55 minutes (of time) away. The comets would have passed thru the OSU telescope beam roughly 55 and 47 minutes (of time) earlier.
The observations of the comet made by the author in 2017 near the position of the “WOW!” object are irrelevant, because that is not where the comet was in 1977 when the “WOW!” signal was observed.
The author does not cite any references regarding observations of hydrogen emission from comets. We have contacted a comet expert and a hydrogen expert and they are both unaware of any hydrogen emission ever having been observed from a comet.
The author does not cite any references of variable emissions from comets. Such variability would have to be incredibly unusual to match the “WOW!” signal. The signal would have to be exactly constant at 30 sigma for 2 minutes since it matched the OSU antenna beam exactly. It would also need to be less than 0.5 sigma for 2 minutes, at a time 3 minutes earlier or later, since it did not appear in the other OSU beam.
The author does not show any specific frequencies in the spectrum plots so it is impossible to compare them with the “WOW!” frequency, or to demonstrate that the comet signal bandwidth is less than the 10kHz bandwidth of the WOW! signal.
Dr. Robert S. Dixon
Director, Ohio State University SETI Program
So it seems that this hypothesis was already excluded before it was tested. This is already briefly mentioned in the article you linked to.
And while it is not clear (to me) what exactly he measured during his observation, it was most likely not the same thing that caused the WOW! signal.