True...but given the mobility granted by Norse ship tech, the ideas may spread quicker than expected. The Norse also have several other military advantages that was revolutionary when first introduced...iron, the horse, the stirrup.
As well as an established culture where surplus population goes out to carve its own jarldom.
A setup where either a stronger Norse colony or a norse/native hybrid culture establishes itself, and parties of young men retain the habit of going a'viking...
They would have several large advantages in power and mobility, and especially after the diseases burned through the natives, could we see a situation where war-parties establish hundreds of "flash-in-the-pan" kingdoms?
Few to none may last for more than 2-5 generations, but it would get tech and diserases spread quickly.