He might be the best in the league when it comes to switching between defense and attack. But what I think people mean is his end product in open play (i.e. goals assists forward passes cut backs and crosses) isn't amazing by the standards of top modern fullbacks, who often need to play like wide 10s on the ball, which is why he's less suited for a lot of teams who might otherwise be in for him.
But he suits us perfectly, since we rely on rapid transitions, and can do a lot more from dead balls. So all he needs to do is use his pace to get in the end of an ambitious through ball or carry the ball up the wing, making the opponent lose their shape or panic, then draw a foul or win a corner/long throw and that's still a "chance created". He can also push right up to the touchline and bank on his pace to get him back in position, which lets us play higher up the pitch and create more of these fruitful transitional moments.
So he does add attacking threat for us, but doesn't suit the way most top teams (including england sadly) play in attacking terms. So he's going to be much more valuable to us than most other teams, and for this reason isn't likely to get sold any time soon imo. Which is honestly maybe for the best, our style of play is built around him, maybe more so than anyone else now that we're without toney and raya, and if we sold him we'd have a lot of adapting to do, even if we used the money well.
Bit of a waffle but that's my 2 cents.