This claim massively overstates the influence of a single text on an entire society.

The culture of knighthood was complicated and could vary substantially depending on time and place, so pinning it all on one text from the 9th century is an argument doomed to failure. Many aspects of knighthood and chivalric culture had little to do with 'cultural synthesis between Christianity and Germanic warrior society'. There were other key moments and key texts that formed the basis of knightly culture, and by looking at a couple of them I hope I can show how knighthood was a concept with a variety of influences.

For something that has absolutely nothing to do with Saxons, let's look at a letter of Fulbert of Chartres to William of Aquitaine, sent around 1020. For some context, William had been fighting against one of his vassals, Hugh of Lusignan, and wrote to Fulbert asking for legal advice. This was the response he got back:

To William most glorious duke of the Aquitanians, bishop Fulbert the favor of his prayers. Asked to write something concerning the form of fealty, I have noted briefly for you on the authority of the books the things which follow. He who swears fealty to his lord ought always to have these six things in memory; what is harmless, safe, honorable, useful, easy, practicable. Harmless, that is to say that he should not be injurious to his lord in his body; safe, that he should not be injurious to him in his secrets or in the defences through which he is able to be secure; honorable, that he should not be injurious to him in his justice or in other matters that pertain to his honor; useful, that he should not be injurious to him in his possessions; easy or practicable, that that good which his lord is able to do easily, he make not difficult, nor that which is practicable he make impossible to him. However, that the faithful vassal should avoid these injuries is proper, but not for this does he deserve his holding; for it is not sufficient to abstain from evil, unless what is good is done also. It remains, therefore, that in the same six things mentioned above he should faithfully counsel and aid his lord, if he wishes to be looked upon as worthy of his benefice and to be safe concerning the fealty which he has sworn. The lord also ought to act toward his faithful vassal reciprocally in all these things. And if he does not do this he will be justly considered guilty of bad faith, just as the former, if he should be detected in the avoidance of or the doing of or the consenting to them, would be perfidious and perjured. I would have written to you at greater length, if I had not been occupied with many other things, including the rebuilding of our city and church which was lately entirely consumed in a great fire; from which loss though we could not for a while be diverted, yet by the hope of the comfort of God and of you we breathe again.

This letter, often entitled 'On Feudal Obligations' or 'The Mutual Obligations of Lords and Vassals' was copied by many other legal scholars of the day and over the subsequent centuries. This letter, and the judgement it made concerning what the relationship between a lord and their men should look like, was foundational to the medieval notion of fidelitas, which we might translate to 'fidelity' or 'loyalty'. Depending on time and place, possessing a strong notion of fidelitas was the most important of the chivalric virtues, and it was almost universally considered one of the most important even when not the most important virtue. Time and time again, we see legal scholars refer back to this letter, and we see knights and noblemen referring to the ideas within it. As an example, the rebellion against king John that resulted in Magna Carta centred around three specific aspects of John's reign: his perceived lack of justice, the harm he did to his men by attacking their property and taking their sons hostage, and his unwillingness to take counsel from his men. We know many rebellious noblemen wrote pamphlets detailing their reasons for rebelling, and the handful of examples we have echo the language of this letter. Of great benefit to historians was one written by Hugh of Lusignan, the rebelling vassal that Fulbert's advice was about, and the ideas within them line up almost perfectly. Fulbert had hit the nail on the head with this judgement, so for the next few centuries people would refer back to it on how noblemen should conduct themselves politically.

Another influential text was Cicero's De Amicitia, a treatise on the nature of good friendship. Where Fulbert of Chartres' letter formed the basis of fidelitas in the political sense, Cicero's treatise formed the basis of fidelitas concerning friends, colleagues and comrades. As examples of ideal friendship, he brought up classical myths that in turn formed the basis of many medieval tales. A particularly common genre was two friends or brothers who go on perilous adventures with each other, often have to save each other, sacrifice things for one another, and in some cases die for each other. The willingness to make such sacrifices, and to treat each others' problems as one's own, was to Cicero the ideal of true friendship. This was an idea that found a lot of currency among medieval writers in tales such as Amis and Amile, Tristan and Isolt, and The Song of Roland. Although not many knights in the Middle Ages actually sat down to read Cicero (though there were some - the prominent 13th century nobleman Simon de Montfort seems to have read quite a lot of Cicero), there's a progression of ideas from classical mythology and Cicero's De Amicitia to the poems and treatises of the Middle Ages, among other influences.

So already I've highlighted two texts that were instrumental in the formation of just one aspect of knightly culture - the importance of fidelitas. That is to say nothing of other chivalric virtues like mercy, generosity, courage, or the Davidic Ethic. These had their own influences, some of which certainly developed from works like Heliand, but also incorporated ideas from other texts. The Bible stands out, obviously, as do classical myths, as do other "Germanic" texts such as Beowulf. Whilst individual aspects of knightly culture can be traced to a handful of particularly influential texts, to say that the whole of chivalry began with one 9th century poem is a somewhat absurd judgement.

Sources:

Beech, George T. "The lord/dependant (vassal) relationship: a case study from Aquitaine c. 1030." Journal of medieval history 24.1 (1998): 1-30.

Crouch, David. The birth of nobility: constructing aristocracy in England and France, 900-1300. Routledge, 2015.

Haseldine, Julian. "Understanding the language of amicitia. The friendship circle of Peter of Celle (c. 1115–1183)." Journal of Medieval History 20.3 (1994): 237-260.

Hyatte, Reginald. The arts of friendship: The idealization of friendship in medieval and early Renaissance literature. Brill, 1994.

Keen, Maurice. Chivalry. Yale UP, 2013.

Mews, Constant J. "Cicero and the Boundaries of Friendship in the Twelfth Century." Viator 38.2 (2007): 369-384.

Reiss, Louise Horner. "Tristan and Isolt and the Medieval Ideal of Friendship." Romance Quarterly 33.2 (1986): 131-137.