Tl;dr: Most Turing Award winners and Decacorn company founders (i.e., exceptionally successful people) don’t attend US top universities, but there’s a fair amount of concentration.

In response to the post Most Ivy-smart students aren't at Ivy-tier schools and as a follow-up to Linch’s comment tallying the educational background of Field Medalists, I collected some data on the undergrad degrees of exceptionally successful people (using some of the  (imperfect) ex post metrics suggested by Linch).

Of the 32 Turing Award winners in the last 20 years, 6 attended a top 10 US university, 16 attended another US university, 3 attended Oxbridge, and 7 attended other non-US universities. (full data)

Of the 97 Decacorn company founders I could find education data for, 19 attended a top 10 US university, 32 attended another US university, and 46 attended non-US universities (no Oxbridge). (full data)

So it seems like people who are successful on these metrics are pretty spread out across both US/elsewhere and elite/non-elite unis, but concentrated enough that having considerable focus on top US universities makes sense (assuming a key aim is to target people with the potential to be extremely successful). 

The concentration gets a bit higher for PhDs for the Turing Award winners (28% at top 10 US universities). It’s also higher for younger Decacorn company founders (e.g., 50% of under-35s in the US at MIT or Stanford) – so that gives some (relatively weak) evidence that concentration at top US universities has increased in the last few decades. 

There’s a doc with more details here for anyone interested. 

[Also for full disclosure: I collected this data as part of my job, not just as a fun after hours project.]