"Also in 867, some of Ragnar's sons, like Ivar the Boneless and Sigurd Snake-in-the-Eye) are the founders of their dynasties (Ivaring and Knytling, respectively). Ivar's descendants are still around in 1066 in western Scotland, and have historically held several lands in Scotland and Ireland, as well as England, I believe.
"The Knytling dynasty reached its peak with Cnut the Great, who as King of Denmark, Norway and England was the ruler of a realm now referred to as the North Sea Empire."
Partly true, but mostly wrong: Ragnar is a legendary king (much more so than Piast), and the stories are wildly different between sources. Paradox clearly uses a Norwagean soruce (but asccording to this source and most others Denmark should already be one country in 867, already in 804 Godfred seem to rule a formed kingdom). One thing is certain: either Ragnar was never a real king (as ruling any specific realm) or he was part of the existing dynasty. So neither he nor his sons are founders of a dynasty.
Ragnar is most likely a ship-king (adventurer in CK terms, in Scandinavia you could be King if you had enough ships). Later sources incorporates him into various royal lines at different times, but never as him nor his sons as founders of dynasties.
The term "knytling" for a dynasti relates to a specific line of Kings, from Knute the 1 (which is slightly schrouded in mystery) over Gorm and Harald Blutooth.
"Svend Estrid, king of Denmark in 1066, is the founder of his respective dynasty."
Both Svend Estrid and his successors are counted among the Knytlings (his son Canute the holy being one of the chief "knytlinga"), in the source where this term is used (the Knytlinga Saga). He is son of Jarl Ulf and Estrid, daugther of Canute the great. The Danes didnt mind skipping through female lines, and besides all his sons that lived to adulthood where bastards anyway (at least 15 or 16 ived to adulthood). There have never been and "Estrid" dynasty.
There are actually some sources for Danish kings in roughly the correct timeframe: like Halvdan ruling in 873 according to a french chronicle, and Horik the Second around 854 to at least 864 (last time he was mentioned).
Also Sigfred ruling roughly from 770 followed by Godfred before 804, would be a better Carolingian start.
Denmark was likely founded as a Kingdom around 700 or so. Ogandus being the first to rule a united Denmark
https://en.wikipedia.org/wiki/Ongendus
We have poor sources 880/890 or so and until 936 when Gorm the Old is mentioned. Possibly Denmark around 870-940 is plagued by rival families and split up Kingdom. Harald Bluetooth (son of Gorm the Old) brings it together, like as a new dynasty, tracing back to a semi legendary Hartheknut, giving name to the Knytlinga dynasty (in principle still ruling Denmark, though a branched of family).