More likely, the nominal values of the y-axis are confidential, and they just want to show the % reduction in traffic. I do assume the crossing with the x-axis is actually at 0.
Of course this could be an innocent mistake made by Niantic. Could be. But given that PR statements in general tend to be manipulative, I'm not willing to trust that. If they didn't want to reveal their resource capacity, they could have labeled the Y-axis with percentages.