Under 49 USC §46506, any crime under 18 USC §113 that occurs on an airplane is prosecutable as a federal offense. 18 USC §113(a)(5) covers simple assault, which can include unwanted touching.

In other words, Davis's own words make him appear guilty. Assault does not require intent.

Now, I wasn't there, and the fact that the FA appears to have reconsidered the matter suggest that he reflected further and decided that it was much ado about nothing. But, a good rule of thumb is not to go around touching other people without permission.