Sure, but many Brits would have taken a similar view of Germans at large when writing an Austrian-set book with significant German characters and everyone speaking German a lot of the time, and where key terms in the schools’s culture like the names of meals, are in German, whatever language is being spoken.
(I mean, everyone behaves as if Verity is being an utter weirdo to refuse to sing or speak German in Three Go, but in reality it wouldn’t have been a particularly unusual position, and a school with strong German, Austrian and Italian affiliations in England during WW2 would have been regarded with suspicion.)
EBD is generally an ‘other cultures are wonderful’ writer, but makes this major exception for Prussians, about whom she and her major authoritative characters are invariably negative, and Prussian characters (Thekla , Frau Berlin) are irremediably awful xenophobes, given to spite, anger and bad language.
It wasn’t an unusual view, no, but it’s unusual in the context of the ‘everyone’s lovely’ CS.
She usually weights the scales on other conflicts. Like the two prominent Irish characters at the start of the series (when the Irish War of Independence had happened only a few years earlier) Deira O’Hagan and Maureen Whatsit of the Saints are both written as Anglo-Irish, UK-loyal, future debutantes, rather than characters likely to have had nationalist sympathies. Biddy, a Kerry peasant from a place that was a hotbed of revolutionary activity, is carefully positioned as the child of a loyal servant of an Anglo-Irish woman and a British army soldier, rather than a rebel..