Putting "Fact" at the end of a sentence doesn't change the fact that you're expressing an opinion. Really winds me up when people do that. I could just as easily go "Jon Obika is the best striker in history. Fact!". Doesn't actually make it a fact.
Here's the "facts", coming from someone who's a big Grealish fan and wanted him to start yesterday. He looks very good in a mid table Premier League side. He is yet to win a major trophy, or play in a Champions League game against those who he is being compared to at the top level. It is much easier to pad your stats when you are the standout star in an average side, and you're relied upon week in week out to carry your side. To suggest Grealish, a very good player, is better/on the same level of De Bruyne, Muller, Bruno Fernandes, Neymar, Salah, Mane is fantastical at best.
To back up my point, and use "stats", FootballCritic currently have Grealish listed as the 14th best winger in the world based on recent form. A decent position, but far from one of the best in the world:
https://www.footballcritic.com/top-players/wingers