I don't know if reddit could run on a single server, but this benchmark does nothing to prove it. A pageview on reddit consist of several http requests. A http requests translates into several requests into a database backend. In this syntactic benchmark he is requesting the same static file over and over again, so nginx and/or linux can cache it in memory (or CPU cache).

Also even if they could put reddit on a single server, it would require a lot of optimization and engineering time is more expensive than hardware. It would also make adding new futures in the future a pain in ass, because you would have to be very greedy with CPU time (and other resources like memory, bandwidth, etc...). Also you want a multi-server setup for redundancy and handling extra capacity.

EDIT: I don't if you also planning to put the database on the same server? Web servers are basically stateless so they are easy to scale. It would be interesting to calculate how much disks that server would need just to handle the IO throughput.

Full disclosure: This is some speaking who used to work for social network with 9M users and 3,000 servers.