Unless you had some very fancy new scans and there has been a shift in knowledge since I learned all about MRI, then I think your neuro is going well beyond what the scans can actually tell him.
A few facts:
MRI does not show all damage due to MS. In fact, the ones that are typically done by the NHS are really pretty rubbish.
Older, more established lesions often show as black spots on T1 scans. However, not all lesions end up this way so the absence of black spots does not indicate that there has been no previous activity.
Lesions visible on MRI come and go. It’s well established that if you scan an MSer regularly, the number of lesions varies. This means that you may have had other lesions if you were scanned before.
New lesions can look different to old lesions, but this is far from an exact science.
So in the absence of black spots, the best way to know if there was any earlier activity is to go over your medical history and find examples of previous events that may have been MS-related. For example, I was diagnosed in my early 30s, but I could think of several things that had happened in previous years, right back to my teens. If there’s nothing that you can think of, then the neuro may well be right: it could have been your first episode.