Just because there is a canonical Floo Powder-making company that has the secret process for making Floo Powder does not mean the process for making Floo Powder is just simply grinding up the floo. It could be more complex than that, and for that reason, we cannot invalidate its canonality. It would be likely there would be other people besides the company had floo as well, but didn't know the correct process to make Floo Powder.
In short, the Floo plant does not equal floo powder. I'm reviving this to being canon.