It seems the dresscode for royal guests was set at formal white daywear (long dress), like the British royal ladies wore, the traditional court dress. For the Coronation Queen Máxima ordered a floor length white gown with her favourite couturier for long robes: Jan Taminiau.
I find it hard to believe that the British court would have communicated a particular dress code to the foreign royal guests and then changed it thereafter at short notice.
Maybe Queen Maxima anticipated by mistake that the dress code would be white long dress and rushed into ordering a gown from Jan Taminiau.