Root rot happens due to lack of oxygen, specifically, which is needed as part of nutrient transport, fwiw. As water is absorbed in a plant, it recedes within the soil. This causes air to get pulled in which delivers fresh oxygen to the root system. Frequent over watering impedes this process and causes the roots to necrotize, which helps feed fungus and mold in the soil, which further erodes the root system and exacerbates the rot.

Root rot usually presents as multiple problems at once since, just as a rule of thumb. This image however was definitely nitrogen. Super common in elder big leaf and younger fast growing plants.

This is correct. Yellowing leaves starting at the bottom and working their way up is nitrogen, which is a mobile nutrient and can be moved from old leaves (bottom) to new (top) in order to support growth.

Root rot was suggested elsewhere in the thread but that typically presents with browning of leaves with no specific locality.