Guido, I think he's saying that the "infinite slope" crossovers prevent the metal cones from being exposed to frequencies they can't handle without breakup and/or ringing. While a more conventional lower order crossover would provide a much lesser degree of filtering of frequencies that are outside of the range the drivers can handle in an optimal manner.
I have no specific knowledge of Joseph speakers, but that is how I would interpret the statement.
Best regards,
-- Al