Yeah, the PRV is open...otherwise there'd be no flow of CO2 from the fermenter into the keg.
In this particular instance I daisy-chained the transfer tubing (and QDs) so the CO2 would purge that, too. I no longer do that--I typically will flush the tubing w/ beer before I connect it to the keg, which does the same thing. I use one of these to open up the end of one of the QDs:
View attachment 602341
I'm also aware that initially, there probably is oxygen in the fermenter, esp if it's a 5-gallon batch, so I'll let the fermenter just burp to the atmosphere for the first few hours of active fermentation, to allow both residual oxygen to exit the fermenter headspace, and allow the yeast to consume whatever it can.
What makes that work better is that I always purge a keg with bottled CO2, which BTW is not pure. So the keg has 99.8 percent CO2 in it in the first place. So I'm squeezing out the last of whatever O2 is in the CO2 stream that originally purged the keg.
I've since tended to move away from purging a keg this way....I just rack into a CO2-purged keg usually. I can't detect any oxidation doing it this way, though one thing I've discovered is that not everyone detects oxidation, nor to the same degree.