The restriction you're talking about does exist, but there's no explicit wording for it. It's described in FAR 61.45(b)(2):
An applicant for a certificate or rating may use an aircraft with operating characteristics that preclude the applicant from performing all of the tasks required for the practical test. However, the applicant's certificate or rating, as appropriate, will be issued with an appropriate limitation.
One of the most fundamental tasks in the Sport Pilot PTS is
maintaining coordinated flight (you won't find it as a discrete task, but if you look at the PTS the phrase "while maintaining coordinated flight" occurs multiple times), and an Ercoupe without rudder pedals relieves the applicant of that responsibility because the aileron/rudder interconnect basically does it for you.
I'm not sure exactly how this works for Sport Pilots, but as you cannot demonstrate to the examiner that you can "maintain coordinated flight" they could reasonably apply the same restriction to your Sport Pilot certificate as they would to a Private Pilot certificate ("Limited to aircraft without rudder pedals"). You would need to demonstrate to a DPE or Examiner that you can operate an aircraft with rudder pedals at some later date to remove the restriction (it is a restriction on the certificate so the only way to remove it is to have the certificate amended/reissued/replaced).
The easiest way to avoid the entire issue is to prepare for and take your practical test in an aircraft with rudder pedals. You can do everything in the Ercoupe and then switch to the Cub for a week to brush up on systems and polish your maneuvers in that plane and get used to using your feet.