That shows he was in the military and was offered to counter the Democrat's claims that he was AWOL when he should have been drilling. None of that is proof he was a natural born U.S. citizen though.
The democrats didn’t ask if he was a citizen, did they?
They asked if he was guilty of a crime under the UCMJ.
They knew he was a natural born citizen because such birth information is included in your Military records especially those of a Commissioned Officer in our Armed Forces.