Regardless of whether it was written by Johnson or Defoe this is the most authentic pirate book ever! I love it.
Authentic in what sense?
'Authetic', as in, was written by a chronological contemporary of the pirates it describes: certainly, and although there are numerous other books also written at the right time (and in some cases written by people actually involved in piracy as participants or victims), Johnson's is just about the fullest.
'Authentic', as in, is filled with accurate accounts of the pirates it describes: that's a whole other debate! In a general sense, some chapters are better than others, some are very good, some are most definitely not. For some chapters it seems likely that Johnson was able to interview credible witnesses - particularly the chapters on Davis, Roberts, Anstis, Phillips, and Lowther. For other chapters, such as Bonnet's and possibly Low's, Johnson appears to have bee working mostly from previously published material such as trial reports and newspaper articles. Some chapters - those on Vane, Rackham, Bonny, and Read, for example - he was probably using a mix of common gossip, newspapers, and his own imagination. Some chapters are highly inaccurate (Every's), others are filled with fictional events and people (Misson, Tew). And, of course, the jury's still out on several chapters.
My definition is that the the author was around at the time, knew some of the characters or spoke to folks who knew the characters, and had some genuine historical / academic interest. As we know from conflicting newspaper reports (even today) eye-witness accounts of the same events often differ (even when there is a lot more actual 'proof' available). As with any human recall "truth" is always subjective. What I love about Johnson / Defoe is that he captures the flavor and atmosphere of the period, relates the tales in the popular imagination, and really transports you back in time.