Gaming benchmarks by preforming better in an older game? Yeah, no :)

A lot of the big games are often optimized on the driver level by both Nvidia and AMD which is why new graphics drivers are released very often compared to other hardware drivers. It's very common to see driver releases the same week as new triple A title that improve performance 40-50% and fix various broken things such as multi-gpu scaling.

In this case it's two games with the same engine (source 1) where the same optimizations happens to work but AMD likely hasn't gotten around to actually verify the optimizations and as such haven't enabled them for CS:GO (Remember steam hardware survey says only ~1% game on Linux so it's hardly worth their time to spend time optimizing Linux gaming)