They either did not look at the history codes in the PCM or they did not scan the vehicle and jusy told you there were no codes because the indicator light wasn't actively on. As soon as that light comes on it sets a code. It is a fault code and the PCM wouldn't turn on that light if it didn't detect an issue. I've run into this issue before with my vehicle brought to a dealership when there was a fault light on the dash for which I took a picture of so there was no dispute. I have a high end scanner and there was a clear fault code stored in memory when scanned, however the dealer told me they scanned it and there was none. It was BS because I scanned my vehicle again after getting it back and the code there clear as day. Bottom line is that they are either liars or incompetent or both. They probably don't want to deal with that right now because the warranty time is much lower than actual book time to do the job. They also probably don't have the master tech/shop foreman available that is required to do the job either. These dealerships like to prioritize $100 oil changes before actual work. It's a very sad situation that most competent knowledgeable techs have left the dealers or the industry entirely because of the horrific pay system and treatment by their employers. Nobody wants to be treated like dirt.
If I were you, I would buy a $100 scanner on amazon( I can recommend the one I use) and scan it for yourself to see if the code is there. If it is not there then they cleared the codes BUT the scanner will tell you when the codes were cleared and at what mileage. Then you will know. I wouldn't let this go until another oil change. I'd drain the extra quart out to trigger the light again.