They may or may not have historical perspective - remember, this is in the far, far future, 'Jetsons' territory or later. 'Our history books tell us of trouble back then' MIGHT mean that they have accurate knowledge of WW2, or it might mean that their general picture of the 20th century is something like our picture of the 5th century BC - some things are known, but not a lot - and that their history books say something generalized along the lines of 'the mid-20th century was a time of turmoil and conflict, with mighty wars being fought'. For that matter, in the confused nature of time travel, that 'turmoil and conflict' may have been CAUSED by their sending the Super-American back in time - and for that matter, it may have ultimately proved the catalyst that led to the formation of their society.