You can hardly call them different generations there is zero difference in the core, the Intel Graphics 617 is also looking to be a pure marketing BS it's the same Gen9 GT2 iGPU as the 615 (we have had benchmarks for it on gfx bench for a while now), maybe it will get a 50mhz more graphics boost clock but even that is doubtful since the results are the same as the 615.
Like it wouldn't surprise me if the whole i7-8510Y thing is literally Apple going to Intel and saying please increase the SKU of the same CPU you gave Google for the Slate so it would appear as like ours is faster on paper.
https://gfxbench.com/compare.jsp?benchmark=gfx50&&did1=64774...
The results are so close that they are both within the margin of error of each other and within the normal thermal/power constraints you have between two different mobile systems.
The challenge with mobile parts is that 25-30% performance delta's are not unheard of between two systems with identical parts on paper but very different thermal and power delivery solutions.