It should be total S in sample analysed. Unfortunately paper did not show any CHNS data. I think you can use Varoon Singh formula and compare results with articles formulae. I am attaching another paper in which CHNS data and % SO3H is given.
You can calculate with the IEC formula is more direct if you want to calculate with a titration method, or could by NMR only you need to know structure, positions of the sulfonic groups.
wmolecularxMolarConc NaOHxVolume comsumption of NaOH/wsamplex1000-wofsulfonicgroupxMolarConc NaOHxVolume comsumption of NaOH