Admittedly, Blizzard does say that content patches get people back. Not as much as xpacs ofc, but they do have more accurate numbers than we do. We only get numbers every 3 months so we don't see when the true increase and decrease occurs.
Made up numbers to illustrate: at end of Q1 you have 10$ at end of Q2 when a new product released you have 9$. Which makes the graph go down. But, what it doesn't show is midway through Q1 you were at 6$ before you released your next product. So, the new product did help numbers even if on the outside it looked like it hurt.
Same thing could occur here, while they lost sub numbers at opening of ToT, we don't know how far they dropped before ToT released before they possibly came back up with its release.
Regardless, we only have sub numbers. What we don't know is how many new players came to WoW compared to how many left.
Tl;Dr: too many variables, what we do know is Blizzard says they generate interest in the game mostly with xpac launches and secondary new content. And we don't have the numbers to disprove it.