Siniat general recommendations are to fix 3rd party products through the Weather Defence board into the supporting metal or timber framework behind which will be installed at maximum 600mm centres.

It is possible to fix in the board only, for this the correct fixing type and number of fixings used must be correctly to install the product and provide the correct level of pull out resistance.

For board only fixings, you would need  to look at specific drywall type fixings and ensure the pull out resistance meets the site requirements. Siniat do not have recommendations for such so your best course of action is to discuss your requirements with your fixing manufacturer. Depending on the designed for windload you may need to increase the fixings above the cavity barrier manufacturer recommendations in order to keep the individual fixing pull out load below the capability of the board/fixing maximum. 

Extract from the Siniat Weather Defence brochure below: