Fakhar's priority should've been getting in, he failed in that. Other than standing in his way, I don't see any reason why a keeper or fielder shouldn't do what De Kock did (if De Kock was doing something other than pointing at all).
To borrow an analogy from football, it is similar to an opposition player calling for a pass from you in a tight situation (which afaik isn't allowed, at least from when I played back in the day). You could argue "oh but he should know the voices of his teammates surely" but in the thick of the moment, it is something that you don't notice. I wouldn't mind this happening if it is in reaction to an obvious unfair advantage gain like in the mankad with the batsman overstepping but is there something like that here?
And in any case, there are clear laws against fake fielding. They've been applied before in such situations (Labu got a penalty of five runs if I'm correct). I'll say what I did for the mankad issue, it was the umpire's fault here thoroughly to not call it for what it was. QdK's behavior could be described with a thousand adjectives from cheeky to smart to unsporting, but the one objective thing that matters is that it was illegal and he got away with it.