The discord username namespace is weird. You get to set a display name which doesn't have to be unique so you can have the name "Bob" and that's what shows up when you message people, but then you also have a unique version on your profile that looks something like "Bob#3922" I'm not sure what happens if 10,000 people pick the same name but as far as I know, there is no limitation to how many people can have the same username.

So it seems almost impossible that this is some technical limitation but rather Discord cracking down on a common deception tactic.