I don't know why they say that exactly. As far as I know, just having Developer Options on but ADB disabled doesn't really do anything. So, my best guess would be it's an easy way to, hopefully, keep uninformed users from making changes to the system that they don't understand. If I were to guess, I think it makes sense from a user support perspective and to cover their butts if someone gets angry because they messed something up.
Far easier to say
Don't enable developer options. It's super unsafe.
Enabling developer options isn't unsafe by itself, but don't touch any of the settings unless you know what you're doing and remember what you changed because we can't help troubleshoot issues caused by changing developer options. Also don't do anything in ADB unless you know what you're doing. Copying and pasting stuff from the internet into ADB is a terrible, terrible idea if you don't understand what you're doing.