WP Username Debate

Hi all. I have had a coder build my WP website for me and he uploaded it via FTP (whatever that is). But in my WP backend in the Username section, he has used a fairly obvious name.

https://www.youtube.com/watch?v=pf548MbsftM at 2 minutes 34 seconds in is what I am talking about. This video shows the username being added in the CP QuickInstall, but I’m assuming it’s the same for my WP backend? I thought we had to choose something that wasn’t obvious? I know I can change my screen username but am I correct in thinking the username in the backend of WP should be hard to guess for security reasons?

Thanks