I'm not sure they said that to qualify their statement or make sure they didn't have to backpedal as much as that it's not fair to compare the sales of a 2 platform release to that of a 3 platform one.It looks like their are some caveats there to me though. They mentioned that it had the biggest launch week of any console game released in 2017. Because they were careful to put the word console in there it probably means something that released on PC in addition to console probably sold better. It's kind of a strange distinction to make when there is still a PC version of Destiny 2 coming next month.
Besides, Activision loves to crow about their sales accomplishments. At least they don't have to REALLY qualify them or say things that don't hold up under scrutiny, like when Gearbox declared that Battleborn had better opening numbers than Borderlands did. Yes we all know that over time Borderlands sold well but it didn't set the world on fire when it was originally released. Also, I think it's safe to say which game had better staying power.