Thank you for posting all the interesting tidbits about comfrey. It seems to be quite an amazing plant.
I have yet to grow comfrey myself but don't be discouraged about the growth of your comfrey plant yet. It looks very healthy. I am guessing that it is an heirloom seed variety. I say that because I have found that many heirloom seeds take their time growing, probably naturally putting all of their energy into making a strong root system. I've noticed plants in the wild do the same thing, sprouts sit forever it seems and then all of the sudden they just explode. I'm guessing it will take off and flourish when it's ready.