Alchemy.Events.on_guild_join

You're seeing just the macro on_guild_join, go back to Alchemy.Events module for more information.
Link to this macro

on_guild_join(func)

(macro)

Registers a handle triggering whenever this client joins a guild.

args : Alchemy.Guild.t

A good amount of these events fire when the client initially connects to the gateway, and don't actually represent the client joining a new guild.