Plenty of people have really high scores like that, They just play Mario Kart ALOT.
Its possible that there is a lone-wolf hacker online, but that is about as likely as Half Life 3 on the switch. I really doubt one of the very few able to hack this game would be stupid enough to risk getting booted off their switch just because they want to win public matches online.
This thread is just some kid whining about losing a game and calling the other person a hacker for it. This is worse than the unending spam of "will there be a 4.1 homebrew" every few days.