Quinn was DFAed this year without ever getting hurt (and still isn't. But he belongs to the Royals now). If memory serves he also managed to play the majority of the team's games in CF (albeit by a very small margin, or maybe it was a slight minority) in 2020 and simply wasn't good enough.
But right now the issue is not who is a good CF, they simply need a spare OF part who can play the position and Moniak is the only person on the 40-man (besides Muzziotti) who can.
But I also think two things are true/clear right now:
The Phillies do not want to give Herrera more playing time (or they would).
They want to follow through on the commitment to give Moniak a longish look (or they'd play Vierling against RHP more). Maybe the light turns on. If not, he'll be be back in AAA soon enough.
The only flaw in all this is that committing to Herrera has a better chance or producing a hot streak, which is why I think it comes back to, they simply believe Moniak's defense is better (he'd have to play more to see if the statistics bear that out).