Kintarou barely beat #14 before. And he doesn't use the clone in the match against Oni.
We don't really know if it was close in the end. It heavily depends on how long Kintarou actually took to beat Vanish (we see him lose 5 points to it, but there are no hints as to how many he loses off-screen, if any), nor how much Hakamada scored outside of Vanish.
Also why would he use clones if Oni doesn't use any moves that would make them necessary?
The last time we ignore Konomi page that was feed to us, some people in this forum don't believe that 10 ball is an actual tier level difference and that fuwa can beat mouri.
10 ball doesn't seem to be, unless you're playing one of the, apparently four, people that can actually split the ball. If Ryoma, Kintarou or Akutsu were to play Yukimura or Atobe, it doesn't seem like 10 ball would be of any use for them.
We also don't know how hard Oni was actually trying during those first three games, maybe he played at the level he and Tokugawa beat Kintarou and Ryoma at at the beginning of the series.
Maybe Oni figured that since he destroyed Kintarou when they played the first time (or that's what it looked like at least), he'd still be fine playing at that level, even if Kintarou improved a bit (and he acknowledges that he underestimated how much Kintarou actually improved).