I was detained by multiple police officers on foot, at a separate location. It doesn't seem right that I could get a DUI when I wasn't even operating a vehicle and never admited to doing so under the influence
You can be "charged" with the crime of DUII regardless of whether at the time of the stop you were driving or not. Being charged with a crime,and being found guilty of a crime are two different things. Since driving is an element of the crime of DUII, you would likely have a very good defense at trial if the police are not able to prove you were driving on the date in question. I recommend consulting with a lawyer with ample experience in DUII law to guide you through the process.