It sounds as if you were not the subject of an institutional action aside from a note in your file that you were accused of a misdeed but not sanctioned in any way. I'd say "no, this isn't an IA" but the best policy is to ask for confirmation from the school by sending the definition from the AMCAS instructions to your school and asking for a reply in writing (email).
That way you can CYA if anyone ever says you falsified your AMCAS application by failing to report an IA.