Yes, the formula assumes a hot (104ºF) spa so there's no way you could stay in such a spa for 2 hours at that temperature. Your body doesn't sweat as much in a warm but not hot spa so the water doesn't build up as many unoxidized chemicals as a result.
Also keep in mind that the formula is for commercial/public spas where they have more stringent requirements. For residential spas, there is no hard rule for when you change your water. If you are cleaner getting into the spa, then you will likely be able to go longer between water changes compared to someone who brings in dirt, lotions, skin oil, more sweat, fecal matter, etc. Chlorine will oxidize some of the chemicals while others will get filtered out, but there are some that aren't oxidized either at all or only very slowly and are not filtered so their buildup can lead to the water getting dull/cloudy or smelly.
With the WRI formula, 2 people for 2 hours 3 times per week would be in a 350 gallon spa:
WRI = (1/3) x (350) / ( (2 people) x (3/7 avg. soak days per day) x (2 hours) x (60 min/hour) / (20 min soak assumption) = 23 days