Ah yes, this again, the reason this was surveyed and published:
Nine-in-10 Democrats correctly identified the statement “President Barack Obama was born in the United States” as factual, while only 63 percent of Republicans saw it as factual.
Applying a strict legal test, the statement about Obama being born in the U.S. is not factual, but opinion.
The evidence that he has supplied to prove the assertion is incomplete and would not pass a strict legal test. Therefore no reasonable person could conclude that he was born in the United States (although to be precise, it is equally opinion to believe he was not born in the U.S.).
Reducing the statement about illegal immigrants being a problem to opinion rather than fact is probably not a very good test of anything, the “opinion” as they call it tends to be a self-evident fact. I would accept it as a fact myself.