They absolutely did not need to hire/pay for two testing teams for both 10man and 25man to spend an undisclosed number of man hours playtesting that encounter before it was ever released.
I imagine some of the 10man team is rolled into the 25man team but if not, that's 35 salaries to pay who knows how many hours to playtest for each raid tier. Not counting the salaries of the team (read: more than 1 person) that creates and balances all the damage numbers for each ability the boss has and spreadsheets/sims that and the healing required/tank damage mitigated/avoided all out to determine what raid composition it takes to complete the script.
It's not like just 1 button pusher making minimum wage decides, "hey, let's just add two more blobs to Immersius and call it a day! Jim, you want to go to Subway for dinner?"