There's no way they sit Burrow if he's 100%. Just the fallout, alone, would make it prohibitive. Imagine losing with Flacco while Burrow was healthy. Lose with Burrow? There'll still be some Monday morning coaches yapping if you lose, but it's much better than the alternative.
Now, what would actually be better as far as on the field? First, are we assuming Flacco keeps this up? He's certainly not going to throw for 300/3 every game. I mean, Burrow doesn't do that. The only question would be if Burrow could overcome any potential rust in a critical game. For me, that's not nearly enough of a concern to sit him for a guy like Flacco. With the mid-December timeline in mind, a week 16 return against Miami would be ideal to let Burrow get tuned up.
What if Flacco goes on a record-breaking streak? Well...I guess that could change things, but the chance of that happening is slim to none and Slim left town.