He didn't? I didn't watch the Championship so your statement can very well be true, but I remember before the first game of the season after Villa had just gone up, that the commentator (who is an avid Norwich fan) kept raving about Grealish saying stuff like he might be the best player in Championship history (or something similar). Thought it was a bit annoying at first, but Grealish was unplayable that day when Villa shocked Spurs.
Whilst I do agree that he hasn't showed much until the last couple of seasons, I think you also have to bear in mind that players have different trajectories. Sancho's, Mbappé's and Haaland's numbers are insane for their age, but they could also fizzle out into nothing when hitting 25-26 (though I really much doubt that). As for Grealish, I think he has really benefitted from spending his early professional years improving his physique in the Championship. I remember when he first broke through and people were saying he was a huge talent. All I saw was a skinny lad trying to use the same playing style he got now, but he mostly got bullied off the ball which looked a bit comical at times, which is in contrast to those previously mentioned. Now most opposition players can't handle him. Wonder how he would have been viewed if Spurs got their hands on him for £20-25m back when he was in the Championship.
I think people here get too hung up on the one poster's quite frankly OTT claim of him being a "generational talent" and "new Ronaldinho" and this has soured the debate a bit.