It's an outdated rule, from when Discussion was more debatey. I never upheld that rule during my tenure as discussion mod and I'm fairly sure Thorn, my predecessor, didn't either (barely anyone posts in Discussion as it is, upholding that rule would only mean another barrier to posting). Probably, I should've deleted it before I quit.
As Icky said, you can call people out on their facts if you don't believe them and ask for sources as needed.
Definitely don't start using the report button on people for not following this defunct rule tho.