The reason we know it isn't a false accusation is because both...

As to the 'harsh laws' this is refering to the 28 Jew Laws of Erwig (680) which are revisions of the Laws of Sisebut (612) and Recceswinth (652).
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In essence the Visigoths were far too nice to the jews and the jews simply betrayed and then murdered them.
Sound familiar?
Because it should.