I checked with Samsung UK technical support and they seem to think that placing these particular rear speakers on their sides should NOT pose a problem with the drivers or other functionality. It might not be necessary after all but just checking in case.
How much - if anything - do rear speakers contribute to the Atmos effect? Many high-end soundbars like Sony HT ST5000 or Sennheiser Ambeo are supposed to be very good with Atmos and yet neither has rear speakers.