Thanks for the photo; it is instructive. For sure, the shortest stem has died and can be cut off at the soil line. Check the bottom of the other stems to make sure that they are not soft.
Your plant is in a very large pot with heavy potting soil. That makes inadvertent overwatering much more likely so your diagnosis is correct. Moving it to a smaller pot would do even more damage to the roots so I advise against doing that.
Do remove any excess soil from the top that is not in immediate contact with the roots below. That will allow oxygen to penetrate into the root zone more readily as the soil dries out. I suggest letting the top inch of the remaining soil to get dry to the touch before adding just enough water that it reaches that same level of dryness again in about a week.
It may take a while and some continued decline for your plant to slowly stabilize and recover from the root suffocation it has already experienced.
