So, after watching a couple 1v1 streams today: They do.
Manually taking control of a player will obviously mess it up a bit, but generally the CPU teammates will try to have one up top, one in the middle and one on the bottom, and one staying further back centrally. And they'll default to the position they started out at on kick-off, unless it's taken up by the player-controlled character.
That means sweeper Waluigi is back on the table. The winger roles aren't quite as differentiated as the defender one, since they play as high up as a striker would, but they probably can still make better use of speed than the CF. My most pressing unanswered question now is whether there's any room for playmakers with a high passing stat. Given how small the pitch is and how easy it is to score from the halfway-line, pumping points into Strength/Speed + Shooting seems the most efficent approach.