It's less about Go being incredibly fast than about Ruby being anomalously slow. It makes sense if you start from the presumption that Go is the only "fast" language they're considering.
Frankly, if you're going to be processing requests and shuffling them to S3, Ruby is nowhere near being the limiting factor unless you do something pathologically bad.
Having spent time recently optimizing a setup that does pretty much exactly what they're doing (in Rails; I don't like Rails, but in this case Rails is not a problem), the time saved doing things like eliminating unnecessary buffering all over the place (e.g. POSTs gets buffered by pretty much everything that likes to consider itself a web server, sometimes multiple times; if you e.g. run Nginx in front of pretty much any Ruby web servers running Rails, worst case you may end up with things passed through at least 3, possibly 4 buffers).
Having spent time recently optimizing a setup that does pretty much exactly what they're doing (in Rails; I don't like Rails, but in this case Rails is not a problem), the time saved doing things like eliminating unnecessary buffering all over the place (e.g. POSTs gets buffered by pretty much everything that likes to consider itself a web server, sometimes multiple times; if you e.g. run Nginx in front of pretty much any Ruby web servers running Rails, worst case you may end up with things passed through at least 3, possibly 4 buffers).
Basically they should be IO bound.