> Það sá Leifur að hún mundi kunna fleira en fátt eitt.
> This leif saw as she would know more /be wiser than, a lesser one

"Leifr saw that she ('mundi' = most likely, in all probability) knew
more than just a little." That's to say, she knew a lot; the
implication being that she has supernatural powers.

> Leifur kveðst eigi kunna að gera hertekna svo stórættaða konu í
ókunnu landi "en vér liðfáir."
> Leif declared for himself (for his part) he knew not to abduct a
woman(Z) thus noble/highborn in an unknown land "for we are short of men"

'kunna' + inf. = "to be able to" (Z 10). "L. said that he couldn't
abduct such a well-born woman in a foreign/strange land, 'with us
(being) so short of men / as we are so few in number.'"