Knut den harde - Mention in Ragnarsona þattr

I was reading the "Tale of Ragnar's sons" for another reason today, and found this piece:

Sigurd Snake-in-Eye married Blaeja, the daughter of King Ella. Their son was Knut, who was called Horda-Knut, who succeeded his father in Zealand, Scania and Halland, but Oslo Fjord broke away from his rule. Gorm was his son. He was named after his foster father, the son of Knut the Foundling. He governed all the lands of Ragnar’s sons while they were away at war. Gorm Knutsson was the biggest of men and the strongest and the most impressive in every respect, but he wasn’t as wise as his forebears had been.

In Old Norse:

Sigurðr ormr í auga átti Blæju, dóttur Ellu konungs. Þeira sonr var Knútr, er kallaðr var Hörða-Knútr, er ríki tók eptir föður sinn í Selund, Skáni ok Hallandi, en Víkin hvarf þá undan honum. Hann átti þann son, er Gormr hét. Hann var heitinn eptir fóstra hans, syni Knúts fundna. Hann helt allt land af sonum Ragnars, meðan þeir váru í hernaði. Gormr Knútsson var allra manna mestr ok sterkastr ok inn mesti atgervimaðr um alla hluti, en ekki var hann svá vitr sem verit höfðu inir fyrri frændr hans.

http://www.germanicmythology.com/FORNALDARSAGAS/ThattrRagnarsSonar....

So now we know at least where the idea that Knut was the son of Sigurd Snake-Eye comes from.
(Not sure what the scholarly opinion of the age of Ragnarsona þattr is).