For example, in that article, author cites 13th century English chronicler that tried to justify the massacre on Scandinavians in England:
The Danes made themselves too acceptable to English women by their elegant manners and their care of their person. They combed their hair every day, bathed every Saturday, and even changed their garments often. They set off their persons by many such frivolous devices. In this manner, they laid siege to the virtue of the married women and persuaded the daughters, even of the nobles, to be their concubines. (135).
Or another quote from it:
After the Viking sack of the monastery of Lindisfarne in 793, the scholar Alcuin (d. c. 804) wrote a number of letters to English kings denouncing those Christians who had begun dressing and caring for themselves as the pagan Vikings did since this had obviously incurred God’s wrath. The Viking raids in Britain*, he claimed, were a punishment from God for the people’s sin of self-care apparent in their emulation of the Vikings:*
Consider the dress, the way of wearing the hair, the luxurious habits of the princes and people. Look at your trimming of the beard and hair, in which you have wished to resemble the pagans. Are you not menaced by terror of them whose fashion you wished to follow? (Somerville & McDonald, 187)
Is it true?
First off, great choice in books! I'm biased, because Angus and Andrew were my undergrad profs, but they do excellent work in general!
Second, yes, that is absolutely a justification used by contemporary and later scholars for the attacks on Christian sites by viking raids. Alcuin, you need to remember, was in the court of Charlemagne at the time, right in the middle of an early Christian renaissance, and the entire Carolingian Empire was going hard for that Roman Aesthetic, which included being clean-shaven.
"Wait, /u/EyeStache," I hear you say, "but Alcuin was giving them shit for trimming their beard and hair!" Not so, says I! Alcuin was giving them shit for the way in which they wore their hair, i.e. the hairstyles.
We know from numerous grave-finds that early medieval Scandinavians carried various combs, nail-picks, and ear-spoons to help keep themselves looking neat and clean, and that beards were an important social indicator in Scandinavian society. In Brennu-Njáls saga the titular character is referred to derisively as being beardless, and his sons as being "dung-beards," because clearly since their dad couldn't grow a beard they had to fertilize their own beards to get them to grow.
Because beards and hair were such an important element of social interaction, then maintaining them is equally important, so Alcuin was saying, basically, "listen, guys, if you insist on acting like pagans, what with your beards and long hair, then of course God is gonna let them kick your asses." Logical? Not quite, but it definitely had some weight to it.
I would definitely recommend you check out Ashby's article in Internet Archaeology 42 for more information on how beards and hair in general were seen in the middle ages, and give Njáls saga a read - you can buy English translations pretty much everywhere, or get it for free at Saga DB for an example of how beards were a pretty key component of social standing in medieval Iceland.
Also, Carl Phelpstead's fantastically titled "Hair Today, Gone Tomorrow: Hair Loss, the Tonsure, and Masculinity in Medieval Iceland" is a great piece as well.