Couplers are not directly related to the type of coach or locomotive. They are independent of the type of rolling stock and most railways end up choosing the coupler that suits their requirements. Just like how ICF coaches have variants with Screw couplers and variants with CBC couplers, there is nothing stopping IR from having LHB coaches with screw couplers. LHB coaches have CBC couplings simply because IR decided to start using CBC couplers. The CBC coupler has got nothing to do with the design of the LHB coach.