The OP holds an ATP with an instrument rating that was presumably obtained in helicopters. Instrument ratings are category specific. That's why there are separate Instrument Airplane and Instrument Helicopter ratings. Otherwise it would just be ""Instrument Rating" (generic).
Let me ask this question, does the ATP certificate also state: Private privileges, ASEL; Instrument Airplane?
Lastly, If the OP doesn't have an Instrument Airplane Rating, Can he legally file and fly IFR in a single engine aircraft now, using Private Pilot privileges?