Oo this is a great question I had never thought of before!

Cater's line in both the manga and the game is “連携攻撃が超強力”, which just means that their coordinated attacks are really strong. (“Word is, they work so well together on the field that other teams don’t know how to counter them.”)

There seem to be three theories:

1) There is a thread on twstsoku from January 2022 of people guessing that, since we don’t really know the specifics of how Spelldrive is played, it is possible that it involves players on the ground.
https://twstsoku.com/archives/88azmyhx.html

A lot of us probably imagine something like the broom-based sport of another certain magic school when we imagine Spelldrive, but it is possible it involves players who aren’t on brooms.

When explaining how the game is played in Book 2, in fact, no one mentions the necessity of brooms, just that “You need magic to play. Not only do you move the disc with magic, but you use it to attack and defend, too. The flashier the magic, the better—it’s how players show off their skills!”

Also, despite Leona being the described as a top-level Spelldrive athlete (after Malleus) to the point that he is recognized during Halloween as a famous Spelldrive player rather than a prince, in Book 6 we learn that Epel is said to be a better flyer than Leona.

This may insinuate that Spelldrive prowess might be less dependent on flying than it seems.

While Jack is repeatedly described as an adept Spelldrive athlete, for example, I wasn’t able to find anyone commenting on his flying abilities, and he is in the track-and-field club.

If there are athletes on foot in Spelldrive games, it is possible that Jack is one of them.

This would seem to be backed up by the rhythmic game in Book 2 where Ace and Deuce play against Leona’s team on foot, using magic against broom-riding players (they are also on foot in the manga).

But this scene does not depict a “real” Spelldrive match and it is equally likely that, having just started their first year at NRC, there might be a rule against Ace/Deuce/Grim riding brooms without teacher supervision, or maybe they just don’t have brooms on hand in the stadium to use because they are not official Spelldrive players (unlike Cater, who represents Heartslabyul in the games, which would be why he would have a broom available), etc.

It is possible that Leona’s team is allowing an exception to the “must be on a broom” rule in Spelldrive (if such a rule exists—we do not know!) for this scene.

There is also Leona himself, leaping in from off-screen without a broom.
He might have jumped off his broom off screen (which would be difficult to animate), but he still dismounts mid-game in order to defend the goal, insinuating that it might not be necessary to remain on one's broom the entire time.

2) Also mentioned in the twstsoku thread is Cater specifically saying that Floyd and Jade’s “coordinated attacks” are what make them so strong.

While the rhythmic again might not be strictly following the usual rules of Spelldrive we do see Ruggie sharing a broom with someone at some point, so maybe the twins are able to coordinate their attacks with one another while being flown by others, or Jade on Floyd’s broom?

3) If brooms are a requirement in Spelldrive it is possible that Jade just spends entire games hovering nearer to the ground than other players, and his “coordinated attacks” with Floyd via magic are really just so effective that they are able compensate for his poor flying skills through teamwork.

While Floyd may also be a poor flyer, he says he is better than both Jade and Azul.

Octavinelle is said to be more on the academic side of the spectrum than the physical side, so it is also possible that Floyd and Jade are simply the top athletes that the dorm has even when their lack of flying ability is taken into account (and Leona does not seem to consider them enough of a threat to send Ruggie after them).

After the tournament, all we know is that Diasomnia came out on top and Scarabia came in last.