He's using 'su' to switch to desktopuser, that's not necessarily, and quite likely not, an account with root privileges. 'su' stands for 'switch user', it's not just to become root.
I use a script in a root terminal to control a lot of my workflow. One thing is that I start browser windows from it when needed. I start them with
sudo -u desktopuser chromium example.com
sudo -u desktopuser firefox example.com
I thought that is relevant to the question "Why bother with Chrome".
desktopuser of course is the normal user which would also start Firefox if I did it via the Desktop gui.