Maybe it did fit - But DM should know better.
But I have a roll aboard computer bag.
Fits perfectly under the CRJ seats, if I pick the right one.
Have had a few agents say you need to pink tag it.
I always smile as they put the tag on the top of my handle.
I then ignore the bag drop and walk onto the plane with my bag.
If you are a DM or even a PM that is how you handle it.
If you are DM, then by now you should know that arguing with GA is a waste of time. You don't argue - You just ignore.