First: EventMachine is not a ruby binding to libevent. EM is a
reactor-based network communication library. See it as Twisted Python
for Ruby. EM doesn't even use libev / libevent underneath because of
cross-platform compatibility concerns.
Second: EventMachine is a Ruby extension written in C++ (and being
ported over to C). There is a pure ruby version of it as well, but
that's for those who don't want to deal with extensions and of course
don't need the speed of C++. I put this because the page seems to make
the assumption that EM is written purely in Ruby.
Jason