They can't use more they can only ever use what is available, the same amount of capacity is available.
Without an enforced cap you automatically share the pipe and get what you get.
With a cap you share the pipe and get what you get up to the cap and then it throttles.
You can never get more than the available capacity, and not being capped wouldn't cause the users who were capped before to get slower speeds, only the users who weren't capped before will get slower speeds.