The World Government is the one that give that title legitimacy
Yonko title is basically shichibukai title on steroids with more territory and less mobility
But the WG doesn't control the Newspaper
The most they can do is pay Morgans off and pray he writes exactly what they want him to write
Morgans wanted to write "Luffy is the 5th emperor" and no one could stop him so he did.
Does the WG agree with that? Obviously not, but their opinion on his newspaper is irrelevant
To the masses at that point Luffy was probably seen as an emperor or a great pirate at the very least
Mind you, Luffy at that point hadn't done anything deserving of the title... at all. He beat 2 commanders, stole a poneglyph, and clashed with Big Mom. Then he went to Wano and was immediately humbled by a "real" emperor