When the Sum Emperor was involved in the struggle for succession, he also secretly trained a unit of heavy cavalry. He was very clear in his mind about how long it would take to train a heavy cavalry unit to be combat-ready.
Lu Chen had only been in North City for a little over a year. If the heavy armor in North City was given to Lu Chen by others and was not crafted by him, and the heavy cavalry of North City was also set up by him, then it would definitely take several years for the North City heavy cavalry to become battle-worthy.
Simply having soldiers don armor and then considering them heavy cavalry doesn't necessarily mean they are strong in battle.
In a short period of time, if heavy cavalry are well-trained, they might be stronger than regular infantry, but if the training methods are inappropriate, the heavily armored cavalry might end up being chased around by light cavalry.