I will offer the general definition used for folks who play period hickory shafted golf clubs. It should look, feel, and perform like the equipment of the day/time you are trying to replicate or emerse yourself into. For example, if you use a modern adhesive, to secure the clubhead to the shaft, you can't see it or tell any difference from a true period club of early 1900's (where they did not have modern adhesives). So most hickory players don't object to modern adhesives to secure the clubhead. you could not pick up a club and tell the difference. But if you put a rubber grip on it, it would not look or feel like a club of the period. It would be noticeably different than period-authentic equipment. That is where you draw the line in my opinion. It must look, feel, and perform like the equipment of the day/time you are trying to replicate or emerse yourself into
Archery-wise, for many of us, this would translate to historical times of Native American culture (there are few bow artifacts of pre-contact or even early contact). So I am fine using a bow made with metal tools - it would look, feel, and perform similar to a period weapon. The string is where I think many of us cheat our intentions. It is certainly easier to use modern materials for a string, but to me the bow does not have same feel. I use strings made of rawhide or gut, as this is truer to the period of interest. You will lose a little performance, but interestingly, if brace height set-up for this material, you will shoot a bow that is ultra-quiet (or has been my experience). A natural string is a more difficult path to take for sure.
The sealant is another area where I don't want to compromise. I use bear grease or pitch varnish. A good pitch varnish is an incredible sealant; and has a look and feel that is different from modern finishes. And no doubt these finishes look and feel to the touch (if you shoot a SE American D-bow with no grip) different than modern sealants.
Just my 2 cents. To me, these details matter. For others, it may be no big deal. I am not judging, just expressing my opinion.