Authentic practice has evolved into a snobbery of elite researchers, that their way is suggested by someone in his treatise and therefore they are doing it right.
Felix Mendelssohn is the person who from what I understand started the idea that musicians should perform the music the way the composer wanted and not the way they wanted. This one idea that in the 1830s the idea of the composer being correct means to me that at that time and before there was much more free interpretation of music. This makes sense since a lot of parts had to be hand copied as they were shared and spread around. Mistakes obviously happen in these copies, so the musicians would fix it in unique ways. Not only that, there was someone who made a simplified version of Beethoven symphonies fixing some of the more dissonant chords. This is a good example of non-authentic practice.
If the case really is that musicians make their own versions of music before the 1830s, then authentic practice really is making your own version and changing it however you feel!