I think this is an excellent example of one of the pitfalls that the author is getting at.

(Disclaimer: not a Discord bot programmer.)

Suppose discord_user_id is an identifier like @Spivak#2024 that uniquely identifies the user. When Discord forces all users to change to unique ID's and eliminates the disambiguating numbers (or allows users to change them), then where does that leave you?

(For the unfamiliar: something like this happened. Not sure if those are really the "discord user id" used in bot service though, but suppose that they are for the sake of the argument.)