In the US, we believe people are innocent until proven guilty. The way this number can be 0 is by ICE and CBP doing the bare minimum investigation "Is this person here legally" before arresting, not after.
There are, of course, situations that make that particularly difficult to determine, for example at home birth and homeschooling. But that's a burden for the state to resolve, not the individual.
> supposing a person is likely to be present without authorisation in the country, and is unable to offer any evidence of citizenship, then what?
It's not their burden to show citizenship or authorization. Before being arrested the state should have gone through a process "Is this person a citizen? where do they work? Do they have an SSN? Do they have school records or is there a birth certificate?" These are all tasks for the state to do before arresting someone.
> It seems this is an incredibly small number for such a large operation,
1. It's entirely too large, particularly because citizenship can be determined before arresting.
2. This is only what has been found by journalists, not the sum total of illegal arrests. Like I said, ICE is specifically trying to obfuscate their operations (the article mentions it) specifically to get around thing like FOIA requests to see how often these false arrests happen.
> Could this be improved?
There's almost no way it couldn't be improved.