In this instance I would imagine that for browsing they would use either Qutebrowser or Surf. I run a very similar setup and use firefox, but that's only because I have about 300 tabs open in normal setup :)

Just because they run urxvt per workspace doesn't mean they can't run another program in a specific workspace, though. It's not a terminal-only setup in the same way that the tty is a terminal-only setup.