Since this is a translation, it's worth looking at the original sentence first, which can be found on British History Online:
Vrbis matronæ ipsæ Sabinæ sunt.
There may be more than one way to translate this ("The matrons / wives of the city are themselves / real Sabines") but the comparison shows at least that "Sabines" comes from the Latin text and not the translator's poetic licence.
Leo T. Gourde's 1943 Master's Thesis An Annotated Translation of the Life of St. Thomas Becket by William Fitzstephen contains the following footnote for "Sabines":
The Sabines were an Italic tribe of central Italy and contemporaneous with early Rome. Known for their simple lives and physical courage, they had a reputation for severe discipline and sturdy character.
If this doesn't rhyme well with the preceding sentences in John Allen Giles's translation, we need to look at the Latin original again (emphasis added):
Ciues Londoniæ, vbicumque locorum, præ omnibus aliis ciuibus ornatu morum, vestium, et mensæ, locutione spectabiles et noti habentur.
Giles translated "ornatu" as "genteel", in which the meanings "refined" and "stylish" are perhaps a bit stronger than in the Latin adjective "ornatus", which can also mean distinguished, honored. In addition, "mores" (plural of "mos") means much more than "manners" in the sense of etiquette, it also refers to character and moral rectitude (both combined into a single concept).
With this in mind, the step from being "ornatu morum" to being a "Sabine" in the sense of "exhibiting severe discipline and sturdy character". It is as if Fitzstephen was trying to describe Londoners in a way that would have pleased the moralists of the Roman Republic.