They didn't claim a performance, they claimed a vague delta performance. I wouldn't be surprised if people were using the core count rather than the percentages Apple provided to estimate a performance number. Especially since the M2 family is well benchmarked already.Reduction from a number 10 to a number 9 in the GPU core count does not change the GPU performance they claimed in their advertisements or landing page.
If Apple hadn't provided a core count to begin with, all would be fine. But they did, and it was wrong. You simply can not say that providing incorrect information is without consequence.