Maybe it's just coincidence that a bunch of other people are saying there's rubber banding... I don't know? You wouldn't deny the Wii-U version had cheating would you? Just trying to see where you stand on the issue, I think that people say that's way worse than this is.
I just tested two races on the same track, one where I played poorly, the AI scored 1st at just over 3:00, and another where I scored first at 2:50, and the AI scored second at 2:52. I'm not saying if the AI will always get within a few seconds of you, the videos linked from marc^o^ are encouraging, but it does seem the AI does better to a point or worse depending on how you play. I'm sure it's better than it was on the Wii U, but I still think it's there in this game.
I don't think placing first in every cup is all that much an indication either way, I mean even the tracks where the AI seems to cheat to me, I still place first overall, or tie for second, etc. And I've never played the tracks before or game for that matter. And other people said the harder difficulties aren't really any harder (in terms of AI being better), so I'm not surprised you have first in everything, but congrats. And if you don't think the AI cheats at all (including rubberbanding), then I'll take that under consideration, but I am still skeptical, especially given my little test just now above.