Some comments if you will
First, the vikings did use siege engines, at least in one of the sieges of Paris, I cannot recall which one.
Secondly, there is good evidence that denmark was united already around 700, there is danevirke from 737 that hints at a King of Jutland at least but there are hints that Angantyr (the king Willibrord met in 716) or his descendants ruled more than Jutland. The Kanhave Kanal on samsø, Samsø is an island located in the middle of the realm of Denmark, smack between jutland and Sjælland and north of Fyn, the island is only a kilometre in breadth at the middle but widens at the north and south ends, at the smallest breadth the island is cut by a canal that would allow a fleet in the natural harbour of Stavnsfjord to rebase and intercept shipping on the opposite side of the island very quickly, effectively a fleet based here with such a canal controls the entry to the Danish straits. Hence it controls the Danish isles as well.
Somebody said that Hedeby was in its infancy in 751 AD, this is not so. Ribe was organised and laid out, probably by a King very early in the eight century but Hedeby was only one amongst many coastal markets at that stage (I suggest somebody read Anders Andrén's article on the urbanisation of the North Sea- Baltic Region in "The twelfth Viking Congress" by Bjørn Ambrosiani and...?). And BTW, Birka did not exist in 751 AD, at that stage the central site of Helgö was the trade and administration centre of that region.
In any case, the first danish king we encounter; Godfred (Charlemagne's greatest challenge, according to the Annales bertiani the franks thought and feared that he would make good his promise to loot Charlemagne's palace in Aachen, fortunately for them he was assassinated- probably on Frankish instigation), was a very able king and commander and controlled both Skåne and vestfold/viken in Norway in addition to Denmark as we know it (quite logical as water connected kingdoms in Scandinavia then). He also had a very strong realm that the Friisi and Abodrites as their taxpaying subjects, as well as the northern saxons. We can piece together a chain of even where the Danish kingdom, after the process of unification and centralisation described in Lotte Hedeager's "Iron Age kingdoms/ Danmark i Jernalderen, mellem Stamme og Stat" (it is out in English and danish) was united in the sixth or early seventh century. Meanwhile the merovingians had been weakened and this allowed the Danes to expand their hegemony and subject the Saxons, Abodrites (etc.), and Friisi, as well as the southern parts of Norway, while moving the centre of the kingdom from the eastern isles (Gudme on Fyn and/Lejre on Sjælland) to the southern part of Jutland (Ribe). This peaked with Godfred and when the Franks started expanding again under the Carolingians their interests clashed as described in the frankish Annals. The nephew of Godfred; Horik and his son; Horik II, continued to pursue the same policy towards the Franks of asserting control over the Friisi and Abodrites, though now with a combination of diplomacy and war as described by Niels Lund in the last "Historisk Tidsskrift". After them the dynasty seems to die out and with the personal nature of kingdoms then Denmark fragmented (as in the twelfth century), only with Harald Gormssøn "Bluetooth" was it reunited, probably around 934- 936 AD. he pushed the germans back that had taken the opportunity of occupying south Jutland, centralised rule, beefed up the infrastructure of the realm, and christianised it. On this foundation his son Svend Haraldssøn "Tveskæg" was able to subject the Norwegians and Swedes, and conquer England. IMO, he, not Knud who often suffered defeat in his battles with Eadmund of England, was the greatest danish military commander of that dynasty; conquering England without defeat and defeating Olav Tryggvesson of Norway (a great commander in his own right) was no mean feats.
Apart from that the Swedes might still have been pagan in the first parts of the 12th century.
The gist of this long rant is that we can safely assume that Denmark was united under one ruler in the eight century but that the realm rested on the personal relations/alliances/social network with magnates of the ruler (just as anywhere else feudalism in the legalistic sense that traditional historians have seen is nonsense until the late 14th century at the earliest as shown by Susan reynolds), if the ruler was weak (not militarily succesful so that he could not dispense rewards) or the dynasty died out the realm fragmented. But so it did in the twelfth century.
Kind regards; Palle Rasmussen, stud mag hist, writing my Master's dissertation on the development and nature of the Danish state from ~1 AD to ~1250 AD.
BTW, I can only rarely reply here, for elaborations, literature or counterarguments you will need to email me.