Greetings @CallumCopping ,
In addition to the information provided you by @Chris_Hofer , I wanted to let you know that there currently isn't a way to message specific course sections on the mobile app. The guide you were referencing is actually almost 10-years old, and also seems to come from a specific institution, and not from the official canvas guides. That being said, within the past several years it's definitely possible that they removed this feature, and it could've been removed since it wasn't working correctly or something.
If you are interested in doing so, I recommend submitting a feature idea for the development team, suggesting this as a feature inside of Canvas. Just ensure to specifically include "in the mobile app", just so your idea isn't moved to the will not consider section, as this feature does already exist on the internet browser version of Canvas. https://community.canvaslms.com/t5/Canvas-Ideas/idb-p/canvas-ideas
I hope this helps!
Noah