I contributed to Ruffle. They don't reverse engineer (this is explicitly a no), the knowledge comes from the (poor) documentation and from the open sourced flash virtual machine.

That’s really impressive. And good to see. I assumed all this flash content was just lost. Surely there are lots of bugs or unsupported features but hopefully it continues to be developed.

I suppose another option is to just use a browser tgat does support flash? Seems like some exist.

Ruffle isn't a reverse engineered project, from what I can tell. it seems like they're following some spec, and comparing with the flash player, not decomping it