Always took this that she knew of Wildfire existing (as generally alot of people did), and wanted the Pyromancer's to make it to defend the city (City had less guards to defend, so using a deadly weapon to bolster the defense would make sense).
So, the Pyromancer's made some wildfire, they didn't go grab the old supply of wildfire, which may have been a rumor.
Trying to remember, when was it stated in the show first, that Wildfire under KL existed? Was it Jaime talking to Brienne in the baths? Did Tyrion actually mention this fact ever?
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Ooh, you're right. Cersei knew about how wildfire could do and that they could make it, but she didn't know about the caches that already exist buried under King's Landing.
I believe wildfire being under KL was first mentioned during that bubble bath time with Jaime and Brienne.
Also: I disagree that only the hardcore know who Walder Frey is. He was featured prominently in the Red Wedding episode as the guy who orchestrated Robb and Catelyn's death and had the last word on Catelyn's death too. Everyone knows who is when they see and hear him on screen.