Well, it's all so vague and general that it's hard to say.
There are some statements that are true
"Many Scottish Clans and families trace their origins to Normandy."
"Scribes recorded the name as it sounded, phonetically. It was not unlikely that a person was born with one spelling, married with another, and buried with another."
"The Normans ... were of Viking origin ...."
and there are no doubt others with a grain of truth, for example the potted histories of emigration to Ireland and elsewhere, but these are singularly lacking in details, such as the names of those who emigrated. They are probably standard paragraphs included in all so-called surname histories spewed out by Brigadoon.
I am quite willing to believe that the surname Cassels (with all spelling variants) orginated from the word 'castle', which was imported into the English language from Latin 'castellum' via Norman French. I am perfectly happy to believe that there was a family named Cassels (or variants) in Lincolnshire, but I'd want to see some sort of properly documented source to link them to the Scottish family.
There are other statements I also question. For instance:
"They .... were granted land by David, then King of Scotland ...."
David I, King of Scots, reigned from 1124 to 1153. David II, King of Scots, reigned from 1329 to 1371.
First, you would surely expect a statement like this to specify which King David it refers to and/or when the grant was made? And second, if Brigadoon found a document saying that King David (whichever) granted lands to Cassels, why did Black not find it?
"The most ancient grant of a Coat of Arms found was ...."
Why, if Brigadoon found a grant with all those heraldic details, did they not say by whom it was granted, to whom it was granted (contrary to widespread belief, a coat of arms is personal to the individual to whom it was granted) and above all when it was granted?