Exactly, it's a mongrel language as you would say, or a hybrid language as Hansioux's youtube guy would say. That means no single ancient tribe can claim full credit for it.
It's like surnames: if you share a surname with this or that historical figure, does that mean he/she invented you? Yes, but also no. In fact, mostly no. He/she should share credit with all your other ancestors (hence expiration of "royal" status after seven generations in the Thai tradition for example), and if the custom had been to use the mother's surname instead of the father's at any point along the way, or some other custom had prevailed, or you had been adopted, or whatever, you couldn't just point to your surname and say hey I'm so-and-so's descendant, and in fact you would have people pointing to your name and telling you you're someone else's descendant.
That a certain part of your ancestry has more influence than all the other parts combined is possible but can't be taken for granted, as the pie chart demonstrates.
Or to put it another way, at what point do you declare a mongrel a new breed?
Remember, the Angles and Saxons achieved political dominance on part of a dreadful little island and held it for a few centuries in the ultra-dark ages. Then the Normans pulled the same trick but (arguably) held on longer, even to this very day (arguably). Their influence on the laws and customs was not quite as thorough as that of the previous invaders but was not non-existent either (as Jotham would have us believe), and their influence on the language has turned out to be stronger.
Why then do the Anglo-Saxons get so much credit? The name makes the claim easier. If the country were called North Normandy or something similar, and the language Norman -- or even British -- the legend of the Anglo-Saxon master race would have less surface credibility.
And anyway, Rowland's theory hinges on America as an Anglo-Saxon nation, because the exceptionalism of the "ancient liberty" of the Anglo-Saxons (i.e. the theory that they "invented freedom") doesn't stand up very well to scrutiny, yet being a superpower is supposed to be the true meaning of freedom. (Is that what Jefferson had in mind?)
I believe the America-as-AS-nation theory has enough holes in it already, but feel free to contribute.