According to http://web.archive.org/web/20260317212538/https://support.mo... its

F9 on Windows

Ctrl + Alt + R on Linux

Command + Option + R on macOS

(It uses JS to only show the one for your platform but with view source you can see it mentions all three of these different OSes.)

So I guess the first guy is a Windows user and you other two use Linux.

Probably. When available, reader mode can also be activated by clicking the little "page with text" icon on the right of the address bar.