There is an implicit understanding in theorycrafting about Tsukihime that if these setting elements were ever explored in an officially sanctioned work they would invariably be subordinated to the trappings of Fateshit; Aylesbury, for example, is open to all kinds of WMG about the Dark Six but the fact remains that Rhongobongo pinning down the very fabric of reality is the most probable reason why a world-ending ritual would take place in England's green and pleasant countryside. In awareness of this it is unavoidable to view the context of the 1700 years ago line being a story about how cultists in a quiet English village are trying to resurrect King Arthur by implanting his chthonic mind and transcendent soul in the body of Rhongobongo's wielder as a most peculiar coincidence.
Which is just boring. I'd rather imagine that Altera pwned Crimson Moon in a symbolic triumph of naturalised aliens integrated into our superior culture against subversive aliens undermining human society by converting its citizens through illicit blood transfusions.