This figure is sort of an overclaim imho. If you look inside the paper, the reported figure is 97 FPS actually (vs 135 FPS for 3DGS on their device). This 2400FPS they advertise is for a degraded version that completely ignores the transparency... but the transprency is both what makes these representation support interesting volumetric effects and what makes rendering challenging (because it requires sorting things). Drawing 1M triangles at 2400FPS on their hardware is probably just quite normal.