My understanding is that comfrey, like all other fast-growing green leafy plants, consumes a lot of nitrogen, potassium and and phosphorus. That's why it's not recommended to be directly in your beds, but rather, around your beds and used to make a liquid compost for other plants. I haven't heard any experts claim it fixes nitrogen. It's a bioaccumulator-it pulls lots of stuff out of the ground, but until you kill it, it doesn't release them.
Having learned some of this along the way, I have some plants in the absolute wrong places. Some I've put effort toward trying to kill them, others I just make sure everything around them is getting plenty of liquid fertilizer.