They are not functionally equivalent, unless there are games I'm not familiar with where custom lobbies are published in a list for strangers to join. Normally a custom lobby implies invite only.
Not everyone is interested in a "fair and balanced competitive match" where you're guaranteed to win no more and no less than 50% of the time. I actually find that intolerably boring.
> They are not functionally equivalent, unless there are games I'm not familiar with where custom lobbies are published in a list for strangers to join.
Lots of the mosts played competitive games have that, or third party websites/discords that have links to custom lobbies.