That's a great question, and maybe reveals what some of the disconnect is.
First, we're advocating neither ignoring knowledge skills, nor allowing player knowledge "because a failure might cause inconvenience". We are advocating for using knowledge skills to resolve uncertainty (including whether what a player thinks is true is actually true), and for allowing player knowledge in order to eliminate metagame policing.
Second, "Hey guys I think she's a Lich! I read that in a novel!" is categorically different from "I kill the orc with my sword without rolling dice." The former is an assertion, which may or not be true in this particular game. Stating it aloud does not change the game state; the DM still gets to determine whether or not it is true.
Declaring the outcome to a combat, on the other hand, does change the state of the game world. A closer analogue to the Valindra example would be, "I point my finger at the orc and say 'bang'. Which my patron told me would kill it. So we're good now."
At that point the DM can say, "Surprisingly, the orc does not in fact fall over dead." Or the DM might say, "Was that eldritch blast? If so, I'll need an attack roll." Or even..."Surprisingly...the orc falls over dead." (Which, honestly, would be awesome if it happened just that once.)