I think that the idea with the people who say that he 'can't take the step up' isn't that United is better than Spurs (it isn't at the moment), but the aspiration for United is to win trophies, and Dier isn't that a good player to play for a club that wins trophies. If the aspiration was to compete for the fourth, third or second place, and not win anything, then sure, but if the aspiration is to compete with the top clubs in Europe, then Dier is nowhere near that level. And considering that we have a shitload of money and we want to go to the top, then Dier isn't the right player.
Obviously, he is good enough for a club that competes to go to UCL and that is the level of their aspiration.