Assume 5% can run a marathon (definitely less) and that 2:50 is 90th percentile (definitely higher) and you're still top 99.5%. How is that not an "incredible feat" as the GP called it?

Well, because it was me. I regarded myself as a fair recreational runner, but nothing more.

But that wasn't a general fitness measure, that was a way to weed out weaker contestants by setting a very aggressive standard. That's aiming to get only elite runners.